"We're hoping to get him back at some point this season. Whether that happens or not, I'm not sure," Melvin told reporters, according to the San Francisco Chronicle.
Prior to the diagnosis by Dr. Neal ElAttrache, the A's placed left-handed starter Brett Anderson on the 10-day disabled list with a strained forearm.
Manaea underwent an MRI on Monday and was examined by ElAttrache, one of the country's top sports surgeons, on Tuesday.
Manaea, 26, was placed on the 10-day disabled list Sunday. He is 12-9 with a 3.59 ERA in 27 starts this season, his third with the Athletics. He leads the team in wins, starts, quality starts (15), innings pitched (160 2/3) and strikeouts (108).
Anderson started for the A's Monday night in Houston and allowed five runs on seven hits in 2 1/3 innings for the loss after allowing just two runs in 26 2/3 innings over his previous four starts. This is his second stint on the disabled list this year as he was out from May 19 to July 7 with a strained left shoulder. Anderson is 3-4 with a 4.02 ERA in 13 starts overall with Oakland.
Anderson's roster spot was filled by right-hander Ryan Dull, who is joining the team for the fifth time this season.
Dull has a 5.89 ERA and .296 opponents batting average in 21 relief appearances. He was most recently with the club from July 30 to Aug. 6 and made one appearance during that stint, retiring the only batter he faced on July 30 against Toronto.
The 28-year-old right-hander was 3-2 with a save and a 3.76 ERA and .280 opponents batting average in 22 relief appearances with Nashville. Dull pitched five times following his Aug. 7 option and allowed five runs on 10 hits in seven innings.
The A's have now used the disabled list 25 times, including 11 times for starting pitchers.