The team's physician said Wednesday that Moyer has been admitted to Thomas Jefferson University Hospital and may remain there through the weekend, according to Philadelphia-area media reports.
"He has had some recurrent symptoms, which is not uncommon," Dr. Michael Ciccotti said, according to the Wilmington (Del.) News Journal. "Jamie's a healthy guy. He is in no way seriously ill. And we're taking appropriate precautions so that we can make sure there's nothing else."
Moyer, the oldest player in the major leagues at 47, was hospitalized with a blood infection a week after undergoing the surgery. While Ciccotti said he does not believe Moyer's current issue is related to the blood infection, the left-hander will undergo several tests, including an MRI exam.
"You can re-strain that area [repaired by the surgery] during the recovery or rehabilitation period," Ciccotti said, according to the Philadelphia Inquirer. "That's where the MRI is helpful, because it can help us determine if that's the case, and then that's just a matter of adjusting his rehab and his therapy."
Moyer, a 23-year veteran, went 12-10 with a 4.94 ERA last season.
Information from The Associated Press was used in this report.