OAKLAND -- A.J. Puk’s first Major League start will have to be put on hold.

Puk, the A’s No. 3 prospect per MLB Pipeline, was placed on the injured list prior to Monday’s exhibition game against the Giants with a left shoulder strain. Slated to fill the fifth slot in the starting rotation after showing promise in his first taste of big league action as a reliever in 2019 -- going 2-0 with a 3.18 ERA and 13 strikeouts in 10 appearances -- Puk’s spot will now be filled by right-hander Daniel Mengden.

Puk was shut down for about a month back in March during Spring Training with the same injury, though he was cleared to throw again during the baseball shutdown and entered Summer Camp feeling completely healthy. The left-hander was away from the club on Monday in Los Angeles to meet with shoulder specialist Dr. Neal ElAttrache.

A’s manager Bob Melvin said there is no timetable for Puk’s return at this point.

“It’s frustrating for him,” Melvin said. “He went through this in spring and we thought this period of time off would benefit him. It did to a point, and then throwing in a game the other day, he felt it to the point where we cut off his third inning.”

The A’s feel good about having a pitcher with experience like Mengden in the rotation. The 27-year-old right-hander went 5-2 with a 4.83 ERA in 13 games (nine starts) last season and enters 2020 feeling confident after making adjustments to his windup.

“It’s great to have this kind of depth,” Melvin said. “Mengden looked great. He threw four innings last time out and didn’t give up any runs. He’s tweaked some things in his delivery and looks more aggressive. It’s nice to have that depth, but we don’t want to get too deep into that too early in this season.”

Jesús Luzardo, Oakland’s No. 1 prospect, could be a candidate to fill Puk’s spot if the injury does lead to a long-term shutdown, but he’s still not fully built up as he returns from a positive COVID-19 test and is still likely to begin the regular season in the bullpen. The left-hander threw 30 pitches to hitters before Monday’s exhibition game.

“We’re not going to rush him,” Melvin said of Luzardo. “We targeted him for the rotation but it’s not gonna be there right now. We’ll get him built up so he can start, but we’re not going to push it.”

Weems added to 40-man

Drafted as a catcher by the Red Sox in 2011, Jordan Weems converted to a right-handed pitcher in 2016 and worked hard to get himself on a big league radar. He impressed the A’s in Spring Training and Summer Camp, and on Sunday that work paid off as the A’s added him to their 40-man roster.

“I can’t imagine someone who has been a catcher their whole career and all of a sudden, two years later, he’s in the big leagues as a pitcher, and deservedly so. He earned his way here,” Melvin said. “You hear the feedback from hitters and it’s impressive considering where he’s been and where he’s come from. He was pretty excited about hearing the results yesterday and he should be proud of himself.”

Weems is expected to make the Opening Day roster and will provide another hard-throwing right-handed option out of the bullpen for the A’s.