Left-hander Sean Manaea’s return from shoulder surgery is being put on hold due to discomfort in his side, A’s manager Bob Melvin said Sunday.

“I don’t know if it’s an oblique or he’s just feeling a little bit on the side,” Melvin said prior to the A’s series finale against the Rays. “So we’re cutting back on him for a little bit here to get him healthy. It’s not the arm, which is good.”

Manaea, who had shoulder surgery in September, said the issue is on his right side. He threw 60 pitches in a four-inning simulated game Thursday at the A’s facility in Arizona and was scheduled to make his next rehab outing in a minor-league game.

“It’s just a little discomfort, not pain or anything,” Manaea said Sunday. “Just taking a little precaution and taking it day-to-day.”

Manaea previously said he hoped to rejoin the A’s around the All-Star break. Melvin did not say how this would affect Manaea’s timetable but that “we’re shutting him down for a little bit.”

Meanwhile, top pitching prospect Jesus Luzardo, who’s returning from a shoulder strain, will increase to 75 pitches in his next start after throwing 61 pitches in a rehab start with Triple-A Las Vegas on Saturday, allowing two runs with five strikeouts in 3 ⅓ innings.

Bullpen shuffle: The A’s on Sunday placed closer Blake Treinen on the injured list retroactive to Friday with a right shoulder strain and called up right-hander Brian Schlitter from Las Vegas.

Melvin reiterated that Liam Hendriks will take over closer duties with Treinen out and Yusmeiro Petit will likely handle the set-up role for now.

“What (Hendriks) has been doing recently is getting us (to the ninth), which means he’s coming in with guys on base, so we have to move that around,” Melvin said. “We feel comfortable with Petit in that situation, too.”

Schlitter, 33, had last pitched in the majors in 2015 with the Cubs. He pitched in Japan in 2017, in the Dodgers system last year, and was 4-1 with 10 saves and a 3.89 ERA in 27 games this year at Triple-A.

“I’ve kept doing what I’ve been doing my whole life, just pitching and trying to throw strikes, get people out,” Schlitter said. “It’s just reassuring that it still means something to the people that are able to give me this opportunity.”

Schlitter made his A’s debut Sunday, replacing starter Brett Anderson in the fourth inning. He allowed five hits and a run with one strikeout in two innings.

“I don’t know that there’s going to be set (bullpen) roles at this point,” Melvin said. “It’s going to be more of a day-to-day proposition.”

Matt Kawahara is a San Francisco Chronicle staff writer.