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Arizona Diamondbacks: RP Jimmie Sherfy

Honestly, it's a bit of a head-scratcher that Jimmie Sherfy hasn't already been called up to join the Diamondbacks bullpen.

The 25-year-old has been lights-out in Triple-A, posting a 2.25 ERA and 0.93 WHIP with a 55-to-8 strikeout-to-walk ratio in 44 innings. Armed with a live 70-grade fastball and a plus slider, he's the prototypical late-inning reliever.

MLB.com wrote: "He's just about ready to impact the big league bullpen, with the ceiling to close games at the highest level if he can continue to throw enough strikes."

Colorado Rockies: IF Ryan McMahon

Ryan McMahon saw his prospect star fade a bit last season, when he hit .242 in Double-A and saw his OPS drop from .892 to .724.

It turns out that was just a bump in the road, as he's reclaimed his top prospect status this year with a monster performance in the upper levels of the minors.

The 22-year-old is hitting .351/.399/.587 with 37 doubles, 19 home runs and 81 RBI, and he could be a legitimate X-factor in a super-utility role for the contending Rockies. He's capable of playing first, second and third base and could push Mark Reynolds for the starting first base job if the veteran's bat goes cold.

Los Angeles Dodgers: RP Walker Buehler

It looks like the Dodgers are prepping Walker Buehler for a late-season role in the bullpen as he's begun transitioning to relief work since being promoted to Triple-A.

A candidate to go No. 1 overall in the 2015 draft, he instead slipped to the Dodgers at No. 24 when it was revealed that he would need Tommy John surgery.

As a result, the 23-year-old entered the season with just five professional innings under his belt, but he's returned strong and skyrocketed through the system as a result.

Buehler has legitimate ace upside and there's little question that his future is in the rotation. That won't stop the Dodgers from taking advantage of his potential as a reliever this season, though, and his stuff could prove to be unhittable in short stints.

San Diego Padres: 3B Christian Villanueva

A former top prospect in the Cubs system, Christian Villanueva was non-tendered after suffering a broken leg in spring training that wound up costing him the entire 2016 season.

The rebuilding Padres took a flier on the 26-year-old this past offseason, and it's a move that could pay dividends as he's returned healthy with a .301/.377/.544 line that includes 17 doubles, 18 home runs and 70 RBI for Triple-A El Paso.

It's all about talent evaluation right now for a team like the Padres, and there's no reason not to give Villanueva a look as a potential diamond in the rough.

San Francisco Giants: RP Reyes Moronta

Reyes Moronta announced himself as a prospect to watch last season, when he saved 14 games with a 2.59 ERA, 1.07 WHIP and 93 strikeouts in 59 innings for High-A San Jose.

MLB.com wrote: "If he can throw more strikes and refine his slider into a plus pitch, he could develop into a closer at the big league level."

For now, his 70-grade fastball and improving breaking ball has continued to play in the upper levels of the minors, and he could be ready to audition for a regular bullpen gig in 2018.

All stats courtesy of Baseball Reference and MiLB.com, unless otherwise noted.