The Oakland Athletics have lost one of their top young pitchers to an arm injury. Lefty A.J. Puk, who our R.J. Anderson ranked as the No. 23 prospect in baseball before the season, has been placed on the injured list with a shoulder strain, the club announced Monday. Puk will see Dr. Neal ElAttrache and manager Bob Melvin admitted there is "a level of concern" during a conference call.

"The fact that we have to put him on the (injured list) right away, obviously there's a level of concern. We want to make sure we get this right before we throw him back out there again," Melvin said. "... It's frustrating for him because he went through this in the spring."

Melvin officially named Puk the team's fifth starter over the weekend. On Monday, he said righty Daniel Mengden will step into the rotation for the time being. Oakland's rotation depth chart currently looks something like this:

  1. RHP Frankie Montas
  2. LHP Sean Manaea
  3. RHP Mike Fiers
  4. RHP Chris Bassitt
  5. RHP Daniel Mengden
  6. RHP Daniel Gossett

Lefty Jesus Luzardo, the A's other top pitching prospect, missed the start of summer camp after testing positive for COVID-19. He is behind the team's other pitchers and will open the season in the bullpen. Melvin said Monday they will not rush Luzardo back into the rotation following Puk's injury. They're going to take it slow.

Puk, 25, is developing a bit of an injury history. The No. 6 pick in the 2016 amateur draft had Tommy John surgery in April 2018 and he suffered a shoulder injury in spring training that would have forced him to begin the year on the injured list had the season started on time. Puk was healthy throughout summer camp prior to this new shoulder issue.

Last season Puk struck out 13 in 11 1/3 relief innings as a September call-up. The raw stuff looks an awful lot like a Randy Johnson starter kit. Here's what our R.J. Anderson wrote about Puk before spring training:

During that big-league cameo, Puk relied heavily on his two best pitches: an upper-90s fastball and a swing-and-miss slider. Both of those offerings are plus or better. The rest of his arsenal -- he throws a 90-mph changeup and a curveball -- didn't receive as much praise, and he struggles with command the way more tall pitchers do (he's 6-foot-7). There's a chance Puk becomes a No. 2 starter behind his fastball-slider.

Montas, Manaea, and Fiers form a solid rotation front three. Luzardo and Puk were expected to be the x-factors this season. They have considerable upside -- Luzardo, in particular, was lights out in September -- and could develop into difference-makers quickly. Now Puk is hurt and Luzardo's move into the rotation has been delayed.

Prior to the Puk injury, Sportsline projected the A's as a 34-win team in the 60-game season. Only the Astros (38 wins) project better in the AL West and only the Yankees (37 wins) and Twins (35 wins) project better in the rest of the American League. The A's are a heavy favorite to be in the postseason race this year. Losing Puk and his upside for any length of time takes a bite out of their chances to upset Houston for the division title, however.