The Phillies' opening-day starter in 2013, Hamels entered spring training behind schedule following biceps tendinitis in November. Hamels said after Saturday's mound session, his third of spring training, he doesn't feel 100 percent.
"I believe I threw 35 pitches," the 30-year-old left-hander said. "To my body it felt like a thousand."
Hamels was scheduled to face hitters for the first time in a bullpen session Thursday but that was canceled. Asked whether he believed he could join the Phillies' rotation in April, Hamels was noncommittal.
"I think that's the last thing I'm going to think about," he said. "Ultimately I just want to get back out and get on the mound and see how I'm going to fare there."
Hamels said he isn't scheduled for an MRI. General manager Ruben Amaro Jr. said the team doesn't have any concerns over the structural integrity of Hamels' arm.
Amaro didn't appear to be overly worried.
"The only thing that's a concern for me is the fact that we have to push him back," Amaro said. "The fact that he's throwing, not having any pain or anything like that, this is part of the rehab. Things happen. It doesn't always go in a straight line. It's really more a matter of fatigue. We don't have any issues about his health, as far as his structure or anything like that.
Hamels was 8-14 with a 3.60 ERA in 33 starts last season. A three-time All-Star and former World Series MVP, Hamels is entering the second season of a $144 million, six-year contract, the largest in Phillies history.
Without Hamels, the Phillies likely will open the season with a rotation that includes former Cy Young Award winner Cliff Lee, right-hander Kyle Kendrick and free0agent additions A.J. Burnett and Roberto Hernandez.
Several players are in contention to become the fifth starter.
"Obviously, it's a bummer for the rotation, Cole is one of the best in the game," Kendrick said. "We're going to have to step up. Guys are going to have to step up and pick him up. I think we have the guys to do that. Hopefully, it's not too long. ... We need him back as soon as possible."