MESA, Ariz. — The competition to fill out the back end of the starting rotation has lost an entrant.

Matt Waldron has been sidelined following hemorrhoid surgery and could begin the season on the injured list.

“Had an infection in his rear end and had to have surgery to drain it and make it better,” Padres manager Craig Stammen said. “… He’ll be out week to week, depending on how that incision heals and how he’s feeling moving around.”

Waldron, who began the spring with two scoreless innings on Saturday, is expected to be unable to do any baseball activity for up to two weeks and then will have to build back up. The season begins March 26.

That leaves Walker Buehler, Marco Gonzales, Germán Márquez, Triston McKenzie and JP Sears vying for what is likely one spot in the rotation. (It is possible the Padres choose to go with six starters for a time in April, which would open up a second spot to be won.)

In that he likely will not return to a mound until the latter half of March, placing Waldron on the IL might be the only way the Padres keep him. The 29-year-old right-hander is out of options and, if healthy, would have to be made available to other teams via waivers if he is not on the major league roster.

Castellanos’ work

Nick Castellanos showed Tuesday afternoon that he is still capable of putting together a magnificent at-bat.

Well before that, he did defensive drills at first base, same as he has with infield coach Ryan Goins early most mornings since signing with the Padres on Feb. 14.

After playing third base at the start of his career and mostly right field the past eight seasons, Castellanos is preparing to play first base for the first time as a professional. The veteran of 13 big-league seasons estimated he has never done such a great volume of specific work at a position this early in camp.

“Probably say definitely going over the details,” he said. “There’s a lot of components to first base. But honestly, I’m having a blast doing it. It doesn’t seem like work at all, so I’m enjoying it. Ryan is very good at what he does.”

Castellanos, who could platoon with Gavin Sheets during the season, made his second Cactus League start at first base on Tuesday afternoon and had two hits. That included a double on the 11th pitch of his at-bat in the fourth inning of the Padres’ 6-5 loss to the Cubs.

“That was cool to see,” Stammen said. “… He had a great at-bat there, and it paid off. He stayed in the at-bat, fouled off a bunch of good pitches, made some good swings, got his pitch and hit it off the wall.”

Castellanos had four fairly routine innings at first. His most adventurous play came when he darted back about 40 feet to catch a pop fly in foul territory.

Not that there is really anything routine for him there at this point. Those who have made the transition to first base say it is difficult to change the habit of heading toward the ball and instead running to the base. Castellanos also has to become familiar with where to be when a ball is hit to the outfield, when he is the cut-off man and instances that are more reactionary and will only come with experience.

“I’d probably just say, like, where I’d have to be in game situations,” he said of what he feels is yet to come. “… Just certain things that you can’t really practice. You just have to wait until, like, a game situation, just to kind of be in the right spot at the right time. So prepare the best I can, but also give myself grace to be able to to learn in the game situations.”

Gonzales’ return

Gonzales was more emotional than he expected during the national anthem on Tuesday.

“I just kind of took a moment to appreciate it,” he said later.

The occasion that got to him and caused him to reflect was his first start in more than 18 months.

The 34-year-old left-hander was out of baseball last season while he worked back from the flexor tendon surgery he underwent in August 2024.

He will be looking for something different in his next start after Tuesday began with his first pitch being drilled to the wall in center field and he allowed three more singles and hit two batters in 1⅔ innings.

“I felt like right away I was kind of using the emotion of it and trying to let that fuel me a little bit,” Gonzales said. “And then once I came down, I realized, ‘OK, I’ve got to get back to doing my job.’ But I just was appreciative of clean, healthy innings.”

Said Stammen: “Especially for him, coming off injury and wanting to be back on this stage — I know it’s spring training, but big-league stadium, big-league uniforms — there are gonna be nerves, gonna be emotions. But he did fine.”

Fine will not be the benchmark as he competes for a spot in the rotation. Or in someone’s rotation. Gonzales, who has made 162 major league starts over 10 seasons, has an assignment clause that allows him to sign with another team that will put him on its MLB roster if the Padres decide he isn’t going to be on their big-league club.

“I’m trying to compete,” Gonzales said. “I’m not looking at this as just trying to make it back. I’m trying to go and be the pitcher I know I can be and go out and dominate like I know I can. So, for me, it was a good step. But the first of many, for sure.”