Manaea left Monday's start in the fifth inning after experiencing discomfort and a decrease in velocity. He felt tightness in the same area following a bullpen session in Cincinnati last week, as well as earlier this year in Triple-A Nashville.
"It really stinks. I don't want to miss any time," Manaea said. "I'm trying to have a good first year. It stinks I have to take some time off, but I'm hoping the time I do take off will be beneficial for me in the long run and I'll come back stronger and better."
Melvin said it's a possibility that Manaea will be ready by the end of the 15-day period, and said he could follow a recovery plan similar to that of Sonny Gray, who spent more than two weeks on the DL with a strained right trapezius. Gray threw two bullpen sessions and was scheduled for a rehab start at Class A Stockton before being activated to face Houston on June 5 instead.
"All things were pointing in a good direction for him. This is a little bit of a setback," Melvin said.
Melvin did not name a replacement for Manaea, who was scheduled to pitch on Saturday against the Angels. Reliever Zach Neal made two starts earlier this season and went three innings on Monday. Jesse Hahn, who was optioned to Triple-A on June 10, has made seven starts this season and is scheduled to start on Saturday for Nashville, and could be a possibility. Alvarez made a rehab start on Monday for Class A Advanced Stockton, but Melvin said he's not ready to return as he recovers from surgery on his right shoulder last July.
Mark Chiarelli is a reporter for MLB.com based in the Bay Area. This story was not subject to the approval of Major League Baseball or its clubs.