BALTIMORE --The playoff-hopeful A's were prepared to enter October without lefty Sean Manaea , then they learned they've lost their ace for all of 2019, too.

How much bad news can one team take in one season? The resilient A's, who on Tuesday announced that Manaea is scheduled to undergo arthroscopic surgery on his ailing left shoulder on Sept. 19, are finding out the hard way.

"I think the specifics we'll talk about moreso after the surgery so we know exactly what was repaired," A's manager Bob Melvin said, "but unfortunately, it's another one we're going to have to wait basically a year for, and we really and truly feel more for him."

Add him to a long list of injured pitchers for Oakland. In the last seven months, the A's have sent starters Jharel Cotton , Kendall Graveman , Daniel Gossett and A.J. Puk, the club's No. 2 prospect, for Tommy John surgery, which comes with a year-plus rehab period. Paul Blackburn (elbow) and Andrew Triggs (arm nerve irritation) have also been on the DL for much of the year, and the A's have had to use 14 different starters in turn.

Yet they still have a tight grasp on the second American League Wild Card spot and remain just three games back of the first-place Astros in the AL West entering play Tuesday.

Manaea was the last remaining member of the Opening Day rotation, leading the team in innings (160 2/3) and wins (12) while turning in a 3.59 ERA across 27 starts in his third big league season, which included a no-hitter against the mighty Red Sox on April 21.

"I think the psychology of it, based on the fact we've had so many this year," Melvin said, "it's almost like we're numb to it."

Manaea, 26, was shut down with shoulder tendinitis in late August and originally hoped to return to the mound before season's end. Now he must not only contend with a year-long rehab, but hold out hope that he's not robbed of even more time. Shoulder injuries can be fickle beasts, and not all players fully recover from them.

"He's always had maybe a little something going on in there," Melvin said. "I think if you look at anybody's shoulder that's a pitcher, you might find something in there. Obviously, they found something enough to where it warranted surgery.

"We'll find a way to manage around it, but it's a big loss for not only us but for him personally."

Renowned orthopedic surgeon Dr. Neal ElAttrache will perform the surgery at the Cedars-Sinai Kerlan-Jobe Institute in Los Angeles.

The A's added right-hander Frankie Montas to their crowded roster ahead of Tuesday's opener in Baltimore, giving them 21 pitchers. Right-hander Liam Hendriks is expected to start Wednesday's game as the "opener," as he's already done on four occasions while the A's attempt to piece together their injury-ravaged rotation down the stretch.