The last thing an NFL team wants in minicamp is a serious injury to a prominent player. But that's not the only thing that can cast a cloud over a club's summer.

There's also the specter of contract kerfuffles. Holdouts by star players.

Sometimes, it's a young player who wants a big payday before his rookie deal runs out. Other times it's a veteran looking for one more payday or a change of scenery. Or a star balking at playing under the franchise tag.

More often than not, minicamp holdouts are much ado about nothing—either the player gets the deal he seeks or caves in and reports.

However, in recent years we've seen some drag into the regular season. Joey Bosa's first year got off to a delayed start in an argument over offset language. Rather than play under the tag in Pittsburgh last year, Le'Veon Bell sat out the entire season.

You can bet the rent that NFL general managers are crossing their fingers the holdouts listed here don't devolve into that sort of mess.

Jadeveon Clowney, EDGE, Houston Texans

The first player included also happens to be the most likely one to hold out into the regular season.

It's a situation we've seen before. After five seasons that featured both flashes of dominance and multiple injuries, the Houston Texans chose to use the franchise tag on edge-rusher Jadeveon Clowney. The one-year deal provides a measure of insurance for the team—Clowney has missed 18 games in five seasons and has yet to record a 10-sack campaign.

For his part, he has balked at the franchise tender. The 2014 No. 1 overall pick believes his performance merits a long-term commitment from the team, and since he hasn't signed the tender and isn't technically under contract, the Texans can't fine him.

Per Aaron Wilson of the Houston Chronicle, head coach Bill O'Brien said: "As you know, he's been franchised. He's not here. It is what it is. The situation is what it is. I have every belief and trust that JD is working on his own and getting ready for whenever he does decide to show up. JD has played good football for us. He knows what it takes to be ready for training camp and stuff like that."

The Texans still have about a month to work out a long-term deal, but to date they've shown no inclination they plan to. If July 15 comes and goes without one, we'll find out just how serious Clowney is about not playing on the tender.

After Le'Veon Bell sat out all of 2018 in a similar situation, a precedent has been set that should make Houston's new GM quite nervous.

Robbie Gould, PK, San Francisco 49ers

Yes, that's right—a kicker holdout.

I'm pretty sure that's a sign of the Apocalypse.

The San Francisco 49ers slapped the franchise tag on veteran kicker Robbie Gould after a 2018 season in which the 14-year veteran missed just one of 34 field-goal attempts.

But as ESPN's Adam Schefter reported in April, Gould not only hasn't signed that tender but also broke off negotiations with the team.

"The bottom line is, I'm unsure if I want to play there anymore," Gould said. "At this point, I have to do what's best for me and my family back home."

Gould's agent was no more encouraging.

"At this time," he said, "we are unsure when or if he will report. It will not be prior to Sept 8, at the earliest, if at all."

Per Michael David Smith of Pro Football Talk, head coach Kyle Shanahan is holding out hope that Gould will return:

“I'd much rather Robbie be here and doing everything. I love having him around. But I understand how he feels and what's going on. If that's the way he'd like to do it, no problem at all. All of us want it to be perfect the way we want it, but if he doesn't want to show up until Week 1, it is what it is. I'll take a very good kicker any time, whether it's Week 1 or on third down just one play before we have to kick. Robbie's a hell of a kicker, and nothing's changed on that. I just hope he shows up by the time we play Week 1."

At this point, his status is completely murky.

Damon Harrison, DT, Detroit Lions

Damon Harrison may have left New York last year in a trade with the Detroit Lions, but he hasn't forgotten fans in the Big Apple.

Nor have they apparently forgotten about him.

The eighth-year veteran, who tallied 81 tackles and a career-high 3.5 sacks last year for the Giants and Lions, recently welcomed twin sons into the world. With those new mouths to feed and a pair of seasons left on a contract that will pay him $6.75 million in base salary in 2019, Harrison was a no-show at mandatory minicamp as he angles for a raise.

"Y'all wake up every day and talk about this contract s--t," Harrison wrote. "Giants fans tweet me every day about how much of a bum I am and how they glad I'm gone lol…I'm not bothering either one of y'all..bout to start blocking folks so I can enjoy tweeting supporters again."

Harrison's problem is one shared by most of the players in this article. Since he's under contract, the Lions can start fining him—and those fines add up.

Players don't have a ton of leverage in this situation, but given Harrison's status as a defensive leader and perhaps the best nose tackle in the NFL, it might behoove the team to get something worked out before things become any more contentious.

Duke Johnson, RB, Cleveland Browns

Cleveland Browns tailback Duke Johnson is something of an anomaly on this list in that he doesn't want a new contract.

He wants a new team.

There have been trade rumors swirling around Johnson almost from the moment that Cleveland acquired tailback Kareem Hunt in February, and Johnson told reporters:

"At the end of the day, I understand the nature of the business. I understand his job, [general manager] John Dorsey's job is to do what's best for the team and organization. And again, if that's getting rid of me for a bigger piece and a better piece, then I'm OK with it. Like I said before, my trade request was to meet them at a middle ground. I was put on the trade block a month before I requested a trade, so that's all it is."

Johnson was at mandatory minicamp, but that may have actually made things worse. The distraction is not what a rising young Browns team needs, especially with quarterback Baker Mayfield entering the fray by calling the distraction "self-inflicted."

Johnson's relationship with the franchise is deteriorating daily, and while the Browns need depth behind Nick Chubb until Hunt returns from suspension (eight games), it's nearing a point where keeping Johnson around isn't worth the hassle.

If he decides to skip training camp, that could be a tipping point.

Chris Jones, DT, Kansas City Chiefs

In March, Chiefs general manager Brett Veach told Brooke Pryor of the Kansas City Star the team was working on a contract extension with defensive lineman Chris Jones after his breakout 2018 season.

"There's a lot of time to go before the season starts, and he's certainly a guy that we've targeted and would love to get done," Veach said. "The conversations have started. I wouldn't say they are heating up at a rapid pace, but you've got to start somewhere. We've had two to three of these conversations, and they're getting better."

Well, here we are in mid-June, those talks have apparently gone nowhere, and now Jones has skipped mandatory minicamp.

It's understandable that he wants a new deal. The final year of his rookie deal pays him $1.2 million, a laughably low number for a player who finished third in the league last year with 15.5 sacks.

It's probably not helping matters that just after bringing over Frank Clark in a trade with the Seahawks, the team handed him a five-year, $104 million extension.

With about $24.5 million in cap space, per Over the Cap, Kansas City has the resources to make a deal happen. A Chiefs team with Super Bowl aspirations that's switching defensive schemes needs Jones on the field, and unless you think his 2018 explosion was a fluke, he's not going to get any cheaper.

The longer this drags on, the less sense it makes.

Yannick Ngakoue, EDGE, Jacksonville Jaguars

Like Chris Jones, Yannick Ngakoue of the Jacksonville Jaguars is heading into the last year of his rookie contract. Like Jones, Ngakoue has blossomed into a productive pass-rusher, piling up 21.5 sacks over the past two years.

And like Jones, Ngakoue sat out mandatory minicamp in the hopes of landing a massive raise.

Since 2016, Ngakoue has amassed more sacks than Demarcus Lawrence (26), who received a guaranteed $65 million from the Dallas Cowboys. And more than Dee Ford (25), who inked a deal with the San Francisco 49ers with $45 million in guarantees. And more than Trey Flowers (21), who signed a deal with the Detroit Lions that included $56 million in guarantees.

Ngakoue told ESPN's Adam Schefter, "I will not be attending minicamp, as my contract has not been resolved."

Due largely to Nick Foles' four-year, $88 million contract, the Jaguars have less than $10 million in cap space. They also have more than one young defensive star looking to get paid. Cornerback Jalen Ramsey tweeted, "Imma ask for so much money, they have to put me on lay-away."

Darius Slay, CB, Detroit Lions

"Snacks" Harrison isn't the only Lions defender looking to eat this offseason.

Veteran cornerback Darius Slay—who has two years left on the four-year, $48 million extension he signed in 2016—joined Harrison on the absent list at minicamp.

Slay, who made the Pro Bowl the last two years and was a first-team All-Pro after tying for the NFL lead in interceptions two years ago (eight), believes he's earned a raise with his performance.

When a fan called Slay out for not honoring his contract, he fairly replied, "They don't keep their words when they cut us while we in our contracts."

At first glance, the Lions appear to have the cap space needed to get a deal done—over $23 million, per Over the Cap. But Harrison is also looking for a pay raise, and the Lions may feel like Slay is already being appropriately compensated—his cap hit of $15.9 million in 2018 is the highest of any corner.

According to ESPN's Michael Rothstein, Slay has already forfeited most of a $250,000 bonus by missing offseason workouts. There's a big difference between that and piling up fines for missing training camp or holding out into the season, but for now at least Slay appears dug in.

Bobby Wagner, ILB, Seattle Seahawks

It's not often that a player holding out gets praised by his team for doing so. Of course, Seattle Seahawks linebacker Bobby Wagner hasn't exactly been holding out.

Just as he was during Seattle's recent organized team activities, Wagner was present for the Seahawks' mandatory minicamp. But just as during OTAs, he didn't participate as he seeks an extension to his contract, which expires after the 2019 season.

"He's handled it beautifully," head coach Pete Carroll told reporters. "You know, Bobby's an incredible player in this program, and everything that he does, just his presence is obvious. He's been around for everything. He's been involved with everything, and he's handled it exactly the way he should under these circumstances."

This impasse has lacked the contentiousness that permeated the Earl Thomas holdout last year, but it's also still just minicamp. The smiles on both sides could fade quickly if we move into camp and there's still no deal done.

On one hand, this would appear a no-brainer for the team. Wagner is the unquestioned leader of the defense—one of the last remaining pieces of the "Legion of Boom" that propelled the Seahawks to consecutive Super Bowls.

But they just committed approximately all the money ever to Russell Wilson (four years, $140 million), and after C.J. Mosley's whopper of a contract with the New York Jets (five years, $85 million), Wagner's deal will come in well over $15 million per season.

Trent Williams, OT, Washington Redskins

Of all the possible holdouts on this list, this one has the potential to be the ugliest because it may not be about just money.

It was no secret that Trent Williams—who has two years remaining on a contract that will pay him $11 million in base salary in 2019—wants a new deal. A strong argument can be made that the seven-time Pro Bowler deserves one.

But according to Jason La Canfora of CBS Sports, Williams' absence from minicamp isn't just about money, and he won't be showing up anytime soon.

Williams is reportedly upset about the way the team handled his medical issues in the offseason, and he's so annoyed that he has demanded a trade or a release and vowed not to play for the Redskins.

Head coach Jay Gruden attempted to downplay the seriousness of Williams' absence.

"We have been talking to Trent a little bit here and there," Gruden told reporters. "He is not here [at practice], you are right. As far as holding out for whatever reasons, that is between Trent and Eric [Schaffer] and Bruce [Allen]. Hopefully, we'll get it all situated soon and get him back here."

If this is just about money, one would think something will get worked out before too long. If it's not, the Redskins will be fielding trade offers from 31 other teams.

And their offensive line is in trouble.