PEORIA, Ariz. -- The Padres' bullpen was one of the best in baseball last season, and the entire unit is set to return in 2019. But if there was one deficiency in camp, with Matt Strahm and Robbie Erlin shifting to the rotation, it was from the left side.

They found a solution on Sunday morning. The Padres and southpaw Aaron Loup agreed to a contract for 2019 with a team option for a second year.

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"Once they showed a little interest, I'd heard about the things the [bullpen] had done in the past, and the group they've got, I definitely made sure to check them out," said Loup, who arrived in time for Sunday's workout. "It was definitely something I wanted to be a part of."

In 378 career big league appearances over seven seasons, Loup owns both a 3.49 ERA and FIP. He split time between Toronto and Philadelphia last season, as he compiled a 4.54 ERA. But his peripherals show a pitcher who was better than that mark indicates. Loup fanned 44 hitters in 39 2/3 innings and posted a 3.61 FIP.

Clearly, Loup's strength is getting left-handed hitters out. They posted a .678 OPS against him last season, compared to a .935 mark for righties. Lefty hitters averaged an 83-mph exit velocity against Loup, compared to 90.3 for righties.

As things stand, José Castillo is still the go-to lefty in the Padres' bullpen. But with Brad Wieck recovering from cancer surgery, there was no obvious second option. Strahm and Erlin could both play a role, but the Padres are currently looking for them to help fill out a patchwork rotation.

The club made room for Loup on its 40-man roster by transferring right-hander Garrett Richards to the 60-day injured list. Richards is recovering from July 2018 Tommy John surgery.

Diaz in doubt

Miguel Diaz is out of action, as he undergoes tests on his ailing right knee. The 24-year-old right-hander was shut down during his first bullpen session on Saturday, when he loaded to throw a pitch but halted mid-motion in obvious pain.

The Padres will await the results of those tests before settling on a next step for Diaz. They've offered no timetable for what's being called a "tweaked" right knee.

In 11 appearances last season, Diaz posted a 4.82 ERA with an impressive 30 strikeouts in 18 2/3 innings. He was expected to compete for one of the final spots in the bullpen as the team's long man.

Stewart in the catching mix

Very little has been decided in the Padres' catching battle this spring. Austin Hedges has a place on the roster, but after that questions abound.

Francisco Mejía, the sport's No. 2 catching prospect according to MLB Pipeline, is by no means a lock to make the roster. In fact, if Hedges wins the starting job outright, there's a chance Mejia opens the year in Triple-A -- where he'll be guaranteed regular at-bats. That could thrust Austin Allen into the conversation for the backup-catcher spot. But questions linger about Allen's defense. His best path to a big league roster spot might be as a third catcher/pinch-hit option.

Enter Chris Stewart. The Padres brought the 12-year veteran on board this spring on a Minor League deal. It's been a long time since Stewart has produced significantly at the plate. But over the past decade, he has gained a reputation for his work with starting pitchers and his elite pitch-framing abilities.

"We brought him in here for that veteran presence," manager Andy Green said. "He receives well and he can add something to the club, much like A.J. Ellis did last year. His standing is probably going to be dictated as much by how other people perform in camp, rather than who he is. He's more of a known commodity."

Stewart's profile at the plate isn't promising. He spent most of last season in Triple-A, where he posted a .576 OPS in 47 games. But if he makes the big league club, it'd be due to his abilities on defense and in the clubhouse. If he doesn't, he'll provide useful Minor League depth, and he'd influence a pitching staff littered with top prospects at Triple-A El Paso.

Camp notes

• The clubhouse in Peoria features 70 lockers. There are 70 players in big league camp.

"Most I've ever seen in camp," Green said. "We've got a lot of depth, a lot of competition, and there are a lot of possibilities for constructing the roster this year."

The crowded clubhouse, Green said, could mean a few of the younger pitchers might only get an inning or two in Cactus League play. A few of the younger hitters could only get a couple of at-bats. It'll be up to those youngsters to earn more playing time.

• Erlin and righties Trey Wingenter, Chris Paddack and Craig Stammen threw their second formal bullpen sessions of Spring Training. A handful of other pitchers, including right-hander Bryan Mitchell and left-hander Jerry Keel, faced hitters for the first time this spring.

• Prospect Hudson Potts, a 20-year-old third baseman in his first big league camp, crushed an opposite-field home run against Mitchell. If the Padres can't land Manny Machado this offseason, there are some in the organization who feel Potts could make strides this year toward becoming the franchise's third baseman of the future. He's expected to open the season in Double-A.

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