HOUSTON — After landing on the disabled list with what originally was diagnosed as a left shoulder impingement, Oakland A’s pitcher Sean Manaea received a little more clarity on what’s going on with his arm.

A visit with orthopedic surgeon Dr. Neal ElAttrache on Tuesday in Los Angeles revealed that Manaea’s issue is rotator cuff tendinitis.

It’s a wild card trying to guess how long Manaea, an Andrean graduate, could be out. Every arm is different. Mets pitcher AJ Ramos was diagnosed with the same injury and landed on the 10-day disabled list, like Manaea, on May 28. Ramos has not returned to pitching.

Manaea will be shut down for the time being. The time frame of a potential return depends on when he’s able to begin a strengthening program and later begin throwing bullpen sessions.

With only 30 games left in the season, it seems a long shot to think Manaea can return before season’s end. But A’s manager Bob Melvin said the club is hopeful Manaea can return at some point in September.

The news on Manaea comes on the same day the A’s learned Brett Anderson also would have to go on the disabled list with a strained left shoulder. But Manaea’s injury hurts a little more. He didn’t miss a turn this season, going 12-9 with a 3.59 ERA in 27 starts.

“With Sean being the only go in the rotation the whole season, that one is probably a little more difficult,” Melvin said. “To lose your guy that’s been the ace of your staff for the better part of the entire season, that one’s a little tough to digest. But it’s what we have to deal with and move forward.”

The A’s have overcome plenty of obstacles this season, having already gone through 13 starting pitchers. They were 79-53 entering Tuesday night’s game against the Astros, just 2 1/2 games back of Houston for the AL West lead and 4 1/2 games ahead of the Seattle Mariners for the second wild-card spot.

Daniel Mengden, who held the Astros scoreless in four innings of relief Monday night, will take Anderson’s spot in the rotation. Frankie Montas will get called up and start against the Mariners on Thursday, which was Manaea’s next scheduled start.

“It’s a credit to our front office in creating the type of depth we have,” Melvin said. “Both of these guys have pitched well, and we have no reason to not be confident in them.”