There's a perception around the NFL that the undefeated Pittsburgh Steelers have yet to play a strong opponent. The defense has looked great, but is it great?

Consider their opponents over the season's first five weeks: Pittsburgh's first four wins (with a bye mixed in) came at the expense of the Giants, Broncos, Eagles and Texans—teams whose current rankings in points per game are 31st, 29th, 22nd and 21st, respectively.

They dominated a better offense this past Sunday: the Browns, 12th in points per game and first in rushing yards through Week 6. The Steelers forced Cleveland's quarterbacks into a 15-of-28, 145-yard day. They held the Browns to 75 yards rushing, 1-of-12 on third down and 0-of-3 on fourth down. It was a 38-7 victory.

But more evidence is needed, particularly because the Steelers lost their defensive play-caller, Devin Bush (ACL), in the Cleveland game. The proof can come in Sunday's matchup against the 5-0 Titans in what might be the game of the week. It will feature two of the league's three undefeated teams and a chance to judge whether this Steelers defense is the real deal or has just stifled lesser opponents.

Tennessee's scoring average (32.8) is second-best in the league, while Pittsburgh has allowed 18.8 points per game. Ryan Tannehill has been sacked five times in five games, while the Steelers are averaging 4.8 sacks per game. They sacked Broncos quarterbacks seven times, Carson Wentz five times and Deshaun Watson five times.

"Best front seven in football," one veteran scout offered. His team hasn't and won't face Pittsburgh in the regular season, but he has been so impressed with the Steelers play that he pivoted a conversation about his own team to Pittsburgh's defense.

That front seven includes two All-Pros, tackle Cam Heyward and linebacker T.J. Watt. It has nose tackle Tyson Alualu, defensive end Stephon Tuitt and linebackers Bud Dupree and Vince Williams. And then there's Bush's replacement, Robert Spillane, who'll have to epitomize the defense's strong depth if the unit is to hold onto that scout's assessment.

"The guys on the edge [Watt and Dupree] are the best tandem [of] outside edge-rushers in the league," a pro scout said.

Watt has the second-highest pass-rush win rate as an edge defender, per ESPN, beating his block within 2.5 seconds 32 percent of the time. (The Steelers are second in the league as a team, winning 54 percent of the time.) Watt has yet to force a fumble this season, but he led the league (and set a Steelers record, probably) last season with eight forced fumbles.

Dupree's team-high five sacks this season are as many as some teams have (Panthers and Jaguars). The Steelers lead the league with 24 sacks, and if they continue on that trajectory of 4.8 per game, they will beat the record set by the 1984 Bears (72 sacks).

Another scout agreed that the Steelers' front seven is the best in the league, and he thought they were the best last season as well. He said Watt and Dupree are in the conversation for the top tandem, along with the Chargers' duo of Joey Bosa and Melvin Ingram. 

Baker Mayfield's second interception last week was a prime example of what Pittsburgh can do:

• On 3rd-and-12 in the second quarter, the Steelers rushed four, and Watt came screaming off the left edge, clean around the Browns tight end and tackle dedicated to slowing him down.
• Attacking from the left side, Watt had the best shot at swiping the ball from a right-handed quarterback. But Mayfield had studied the tape on Watt, and he twisted his torso and switched the ball into his left hand just as Watt came full speed for his throwing arm. Crisis averted—for a second or two. 
• Mayfield centered himself back in the pocket, scanning for options and allowing precious time for Watt's counterpart, Dupree, to beat Browns rookie left tackle Jedrick Wills Jr.
• Dupree now raced toward him, forcing Mayfield to escape to his right and heave an unsteady throw off his back foot.
• Cornerback Cameron Sutton took full advantage of the pressure created up front as he leaped high and plucked the ball out of the air.

Pittsburgh's defense is top-three in nearly every statistical category (eighth in passing yards per game), and the unit has powered the team to its first 5-0 start since 1978. The defense has been especially stout against the run, starting in Week 1 when Giants running back Saquon Barkley managed six yards on 15 carries. They are second in rushing yards allowed per game at 66.2.

Now comes a Titans offense powered by Derrick Henry, a running back built like no other.

So how does a team get a ground game going against this defense? The pro scout said jet motion and plays that incorporate misdirection might seem like solutions, but the issue is those plays send running backs to the edges, where Watt and Dupree are both solid against the run. "With that defensive line and you add those two backers, it's a five-man front that is hard to run around and run through," the scout said.

"They mix up their fronts and their calls, so you don't know exactly where they will be all the time," an assistant coach whose team has played Pittsburgh said.

The scout said it comes down to a battle of attrition. Keep running at them even if it's not creating big gains. "You have to know you aren't going to get a whole lot, but you can't be one-dimensional," he said.

The Steelers lost Bush for the season in the second quarter Sunday, prompting legitimate questions about whether they can continue at the same blistering rate.

"[The] Steelers have always had the next-man-up mentality," the pro scout said. "They do a great job of drafting players exactly for what they need. Each time a guy is riding off, they know how to slide a guy right in."

The Steelers didn't actually draft Bush's replacement. Nobody did. Spillane went undrafted out of Western Michigan, but the scout expects the third-year player will be able to adequately fill Bush's void.

When news broke that Bush was lost for the year, the instant comparison was to when the Steelers lost Ryan Shazier in 2017. The team struggled to replace him until they moved up 10 spots to select Bush in the 2019 draft.

One executive said it's not the same scenario because Shazier was playing at a Pro Bowl level when he got hurt, and Bush isn't quite there yet. But he said there is a big drop-off in linebacker talent after Bush. Spillane has been a good special teams player, but can he run and cover like Bush?

The veteran scout still considers the front seven the best in football without Bush, but the executive disagreed with that assessment, even before Bush was lost:

"You can't elevate the status of other players just because Watt, Heyward, Dupree and Bush are good," the executive said. "Fact is, Alualu has been a bust since he was drafted, Tuitt has been an injured bust since he got paid, and Vince Williams wouldn't start on most other teams."

The pro scout said he's been surprised by Alualu's play this season. When nose tackle Javon Hargrave left in free agency, Alualu moved inside from defensive end to fill his role and is making something of a comeback in his 11th season. He was Jacksonville's first-round pick in 2010 and has never lived up to those expectations.

Because of that history, the pro scout expected him to be a base-down player, "a stopgap placeholder that comes off the field on third down," but instead has found him to be a real contributor to Pittsburgh's defensive front. Alualu already has three tackles for loss this season, three passes defensed, three quarterback hits and a sack. In all of last season, he had four tackles for loss, two passes defensed, three QB hits and a sack.

Another player to watch against the Titans is safety Minkah Fitzpatrick. He had his first interception of the season against the Browns, something scouts took note of because it marked a change in how the Steelers deployed him. In previous weeks, Fitzpatrick had been playing mostly single-high, but on this particular play, the Steelers used him as a freelancer.

"They put him down in the box as a robber, and he gets a pick-six," the pro scout said. "So, knowing that's available, I'll be interested to see how much they use him that way going forward. It's a double-edged sword, though, because then [strong safety Terrell] Edmunds has to drop, and that's not really what he brings to the table."

If the Steelers defense dismantles the 5-0 Titans, the league will certainly be talking more about their dominance. "They are for real," the veteran scout said. "They will shut down Tennessee."

Kalyn Kahler covers the NFL for Bleacher Report. Follow her on Twitter for NFL musings and weird quarantine thoughts: @KalynKahler.