Manager Bob Melvin said Manaea could miss most -- if not all -- of the 2019 season, but the A's will not know for sure until after the surgery.
Manaea, who was 12-9 with a career-best 3.59 ERA and 108 strikeouts in 27 starts this season, will visit orthopedic surgeon Dr. Neal ElAttrache in Los Angeles to have the procedure on Sept. 19.
"The specifics we'll talk about more after the surgery so we'll know exactly what was repaired," Melvin said before a game at Baltimore. "Unfortunately, it's another one we'll basically have to wait a year for. You really and truly feel more for him. He was the one guy here for a long time left in the rotation."
Manaea, 26, began strengthening exercises on Sept. 3, but the team felt it was best to have the surgery at this point.
"He was that guy that every series, when teams played us, they're looking to see if Manaea's pitching in the series," Melvin said. "That's what your top guy means to you. Again, I feel worse for him. We'll find a way to manage around it."
When he was placed on the disabled list on Aug. 26 for rotator cuff tendinitis, Manaea was tied for second in the American League in games started, ninth in innings pitched and sixth in fewest walks per nine innings (1.79).
He threw a no-hitter in April against Boston.
The A's are three games behind the Houston Astros in the AL West race and currently are second in the wild-card standings with a 7½-game lead entering Tuesday's action.
"I think the psychology of it, based on the fact that we've had so many this year, it's almost like we're numb to it," Melvin said. "It started in spring training with a couple of guys."
"Everybody feels for [Manaea] and then you just kind of move past it and continue to try to play in the fashion we have. And he's been out a little bit, too. It's not like this is a sudden thing that's happened the last day or two," he said.
The Associated Press contributed to this report.