Trevor Rosenthal will not be making his regular-season debut for the A¡¯s anytime soon.

After visiting Dr. Greg Pearl, a shoulder specialist in Dallas, on Tuesday, Rosenthal underwent surgery for his thoracic outlet syndrome on Thursday at the Baylor Scott & White Heart and Vascular Hospital. The right-hander will remain in Dallas until Sunday and will return for a check-up visit with Pearl in approximately eight weeks.

¡°I had a feeling that was going to happen,¡± A¡¯s manager Bob Melvin said before Thursday¡¯s game against the Astros. ¡°Everything went well. I don¡¯t really have a timetable or anything like that.¡±

The official diagnosis given to Rosenthal by Pearl was severe neurovascular compression. The procedure, which involved the removal of his first rib, a pec minor release and a scalene release to open up a larger space for his arms to move around without pain, can take around 3-4 months to recover from. The right-hander originally landed on the 10-day injured list with right shoulder inflammation just before the start of the regular season.

The surgery is rare in baseball compared to the increased occurrence of elbow and shoulder surgeries in recent years. Other Major League pitchers who have undergone the operation include current A¡¯s starter Frankie Montas and former Oakland pitchers Andrew Triggs and Bobby Wahl, as well as Rangers right-hander Mike Foltynewicz and Orioles righty Matt Harvey.

Rosenthal, 30, signed a one-year, $11 million deal with the A's in February and entered the season with high expectations to replace former All-Star closer Liam Hendriks. After getting a late start to Spring Training due to a groin strain, Rosenthal appeared in four Cactus League games and posted a 2.25 ERA.

Which reliever Melvin turns to for ninth-inning duties is still up in the air. Left-hander Jake Diekman was thought to be the leading candidate to close games for Oakland prior to Rosenthal¡¯s arrival, though the manager indicated that right-hander Lou Trivino could be first in line to get some save opportunities. Trivino entered Thursday having allowed one run and one hit through his first four appearances of the season.

¡°It¡¯s not ideal to use Jake in the ninth inning,¡± Melvin said. ¡°A lot of times, his spot comes up before that in the eighth. If we can keep moving forward with Lou, he gives us an option to close as well. He¡¯s pitched really well for us. We¡¯ve seen what he can accomplish when he¡¯s feeling good.¡±