Every year, a gaggle of highly touted MLB prospects try to prove themselves in spring training.

Every year, a handful of those prospects aren't ready for prime time. 

We're not talking about guys kept down entirely because of service-time machinations or players too young and inexperienced to have even an outside chance of cracking a 25-man roster. Rather, we're looking at elite, advanced MiLB chips (based on MLB.com's top-20 rankings and listed in ascending order) who didn't do enough during the exhibition slate.

Will they be in The Show sometime soon? Quite possibly. Do they require more minor league seasoning? All signs point to yes. 

RHP Mitch Keller, Pittsburgh Pirates

Overall Prospect Rank: No. 19

Mitch Keller had a shot to crack this year's Pittsburgh Pirates rotation after he ascended as high as Triple-A last season. The budget-conscious Bucs are bent on contending and are always ready to promote cost-controlled talent.

Unfortunately for Keller, he surrendered 10 hits, 10 earned runs and three home runs in four spring innings before a demotion to minor league camp.

"He's not the first or the last guy to have some struggles in spring training, but what's really encouraging is his attitude about it going out," Pirates general manager Neal Huntington said, per Bill Brink of the Pittsburgh Post-Gazette. "It's not what he wanted, it's not what we expected, but he's ready to tackle it head-on and go attack Triple-A and earn his way to the big leagues."

The 22-year-old should still get a look at some point in 2019.

INF Brendan Rodgers, Colorado Rockies

Overall Prospect Rank: No. 10

After second baseman DJ LeMahieu bolted to the New York Yankees via free agency, top Colorado Rockies prospect Brendan Rodgers had an opening. 

The middle infielder didn't seize the day. Instead, he went 8-for-36 with just one extra-base hit in big league camp before he was reassigned to Triple-A Albuquerque.

"Offensively—and I told him this—he's improved from 12 months ago to now," Rockies skipper Bud Black said of the 22-year-old, per Patrick Saunders of the Denver Post. "Just in the spring training at-bats, laying off the breaking ball, not expanding the zone, he's improved."

Obviously, Rodgers needs further improvement before he plies his trade at Coors Field.

OF Kyle Tucker, Houston Astros

Overall Prospect Rank: No. 8

Kyle Tucker hit .332 with a .989 OPS at Triple-A and sipped his first MLB cup of coffee last season. While the Houston Astros' outfield depth chart is full, that experience at least gave him a puncher's chance of forcing his way onto the big league roster.

He made the choice relatively easy for the 'Stros by going a decent yet unspectacular 7-for-25 with eight strikeouts in the Grapefruit League. More work is necessary to make his next opportunity last, as Houston manager A.J. Hinch said, per Chandler Rome of the Houston Chronicle:

"He has to keep working on some subtleties to take the next step. His swing improved over the last week, but he did struggle a little bit at times during this camp. His attention to detail on defense improved and we did bring some things to his attention. I told him he's just got to keep working toward making himself a better player so that when he gets the next opportunity, he's ready to perform here."

OF Eloy Jimenez, Chicago White Sox

Overall Prospect Rank: No. 3

Eloy Jimenez came into camp ready to prove himself a new breed of slugger. The Chicago White Sox prospect has the pop of Aaron Judge with lower-strikeout, higher-contact tendencies. 

That might still be true, but Jimenez didn't break out this spring. Instead, the 22-year-old hit .154 in 26 Cactus League at-bats, never walked and struck out nine times. He frequently looked overmatched. 

"I tried to do too much," Jimenez said. "I think that's why I don't have the good results."

After marinating at Triple-A, where he hit .355 with a .996 OPS in 211 at-bats last year, he'll surely get another shot with the rebuilding ChiSox.

3B Vladimir Guerrero Jr., Toronto Blue Jays

Overall Prospect Rank: No. 1

Vladimir Guerrero Jr. entered spring training as the prospect most likely to be kept down solely for service-time considerations, even after he rocketed to Triple-A in 2018 and posted a 1.073 OPS across all levels.

Then, the 20-year-old son of Vlad went 4-for-19 in the Grapefruit League before he suffered a Grade 1 oblique strain March 10 that could keep him out until at least the beginning of April.

No one questions Guerrero's skill as a pure hitter. He's a star in the making.

But his slow spring start and subsequent injury give the Jays all the cover they need to stash him in the minors for the time being...and it's probably the proper call.

All statistics courtesy of MLB.com and Baseball Reference.