PEORIA, Ariz. -- The Padres' deep bullpen took a hit on Sunday when left-hander Jose Castillo exited his bullpen session early with left forearm tightness.

Castillo is still being evaluated, and the club has yet to determine a course of action. But it's undoubtedly rough news for the hulking 23-year-old, who was presumed to be the top left-handed arm in the Padres' bullpen this season.

In 37 appearances last year, Castillo posted a 3.29 ERA. His other numbers hint that his rookie campaign was actually much better than that mark indicates. Castillo struck out 52 hitters and allowed just 23 hits and 12 walks over his 38 1/3 innings. He notched a 2.61 FIP -- a metric designed to remove batted-ball luck from ERA.

Castillo's absence would be a tough break for a unit that ranked as the best in the National League last season, according to fWAR. The rest of the club's late-inning pieces -- namely, Kirby Yates, Craig Stammen, Robert Stock and Phil Maton -- are predominantly right-handed.

Of course, if Matt Strahm's transition to the rotation veers off course, he could return to the 'pen, where he was a lefty weapon last year. Castillo's absence could also present opportunity to the newly signed Aaron Loup, who has posted excellent splits against left-handed hitters throughout his career.

If the Padres are looking for another lefty in their 'pen, Kyle McGrath is a non-roster option. Brad Wieck, who is recovering smoothly from offseason cancer surgery, is unlikely to be ready by Opening Day. But he's slated to throw his first bullpen session of Spring Training on Tuesday.

Margevicius sharp vs. Cubs

Lefty prospect Nick Margevicius is known through the Padres organization as a cool customer. He's a savvy pitcher who rarely, if ever, lets his emotions get the better of him.

"Today, I was feeling pretty calm, until I was on the bus ride over here and I checked the lineup for the Cubs,” Margevicius said. “Seeing [Javier] Baez and [Kris] Bryant and [Jason] Heyward and [Willson] Contreras in the lineup, I got a rush of butterflies real quick. But once I got out there and got throwing, I felt totally calm."

That's no small feat for a pitcher who's made only 10 appearances at Class A Advanced. In the Padres' 6-4 loss to the Cubs on Monday, Margevicius worked two hitless frames, striking out a pair and issuing one walk.

"He's got a good feel," Padres catcher Chris Stewart said. "He just throws the ball where he needs to, and he executes pitches."

Paddack set for Tuesday debut

The Padres have waited a long time to find out how Chris Paddack's stuff might play against big league hitters. They'll get to see it for the first time on Tuesday.

Paddack, MLB Pipeline's No. 34 overall prospect and the Padres' top-ranked right-hander, is slated to start against the Brewers. He's a serious contender for a place at the backend of the Padres' rotation, even though he pitched only 90 innings last season and missed the 2017 campaign after Tommy John surgery.

Between Class A Advanced Lake Elsinore and Double-A San Antonio last season, Paddack posted some absurd numbers. He made 17 starts and recorded a 2.10 ERA, striking out 120 and walking only eight.

The Padres' staff has been extremely impressed with Paddack during his bullpen and batting-practice sessions. His changeup is one of the best pitches in the Minor Leagues, and he's working hard on a curve as well. Plus, Paddack is unlikely to be overwhelmed by the moment.

"His body language, the way he attacks, he's got some confidence," manager Andy Green said.

Paddack's trademark intensity comes with a few quirks. He's known for wearing a suit to the ballpark on days he starts. That applies to Cactus League play as well, he said.

"If you look good, you feel good," Paddack said. "If you feel good, you pitch good."

On the diamond

• Stewart has already made it clear why the Padres gave him a non-roster invite this spring. He picked off Kris Bryant in the first inning, then threw out two runners trying to take second base in the fifth. A defense-first catcher, Stewart is highly regarded for his framing and his ability to work with pitchers. He's competing for the backup catcher job with prospects Austin Allen and Francisco Mejia.

• After Fernando Tatis Jr. tallied his first hit of the spring with an opposite-field single in the ninth, Ty France smashed a long home run to left field. France also singled in the seventh inning, as he continues his strong start to Cactus League play. Of course, with Manny Machado on board, it's hard to envision a path to playing time for the 24-year-old third baseman.

"Obviously, he was at the front of that line of guys that were looking to compete for that job until we signed Manny," Green said. "It's great to see him still go out and do some positive things. He's got a real chance to impact us at some point this year."

Up next

Paddack and the Padres face the Brewers at 12:05 p.m. PT on Tuesday in Maryvale, Ariz. Lefty rotation candidate Robbie Erlin and righty bullpen candidate Trey Wingenter are also slated for their spring debuts.