Hamels, who had already been ruled out for the start of the regular season due to inflammation in his throwing shoulder, told reporters his arm felt "fatigued out" after a bullpen session this past Saturday.
"I believe I threw 35 pitches. To my body, it felt like a thousand," Hamels said. "It just wasn't healing up or getting that response back so I could go out and do it again. I didn't feel like it was safe to push it in that direction because I think that would have led to injuries."
The Phillies do not plan to have Hamels undergo an MRI or receive a cortisone injection, though the setback could conceivably keep the three-time All-Star sidelined until May.
Hamels is coming off a somewhat disappointing 2013 campaign in which the 29- year-old went just 8-14 with a 3.60 earned average in 33 starts.