The left-hander experienced "minimal" discomfort in his left elbow during a bullpen session Thursday and won't throw for the next six days. He'll likely need at least two bullpen sessions before he can rejoin the rotation, meaning it could be close to two weeks before he makes his next start.
"It's definitely unfortunate, but you can't do anything about it," Hamels said. "I'm just going to try and progress and get better, because the only way I'm ever going to help this team and this organization is when I'm healthy."
Hamels, who leads Phillies starters in wins (14), innings (167 1/3), ERA (3.50) and strikeouts (156), has been on the disabled list since Aug. 22 with a left elbow strain. He was scheduled to throw 45 pitches at "the intensity of a game" Thursday, including curveballs for the first time since the injury. But it was a fastball that gave Hamels the first sign of a problem, and he cut the session short after 25 pitches.
"I was holding back and it was changing my delivery," Hamels said.
Hamels has already thrown 35 more Major League innings than last season, and is 14 away from his professional high of 181 1/3, set at three levels in 2006. Still, the Phillies feel this most recent setback is cause for alarm.
"There's nothing structurally wrong; his ligament's intact," assistant general manager Ruben Amaro Jr. said. "We do not believe this to be anything long-term."
Hamels said the injury likely won't heal completely until the offseason. He said he would have been able to make his scheduled start Sunday against the Marlins, but questioned how effective he would have been. Instead, the 23-year-old will now wait six days while his new anti-inflammatory medication runs its course, and Adam Eaton will take the mound in his place on Sunday.
"If I don't have that confidence that I'll be able to help the team out enough, then I don't think it's worth it," Hamels said.
Given the Phillies' recent surge in the National League East standings, the decision to hold Hamels out was not an easy one.
"But at the same time," Phillies manager Charlie Manuel said, "it doesn't matter what the situation is -- if he's not healthy, there's no way we want to send him out there. He's had enough problems as it is.
"His future's at stake."
Tom Keller is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.