Lee, who signed a five-year, $120 million contract with the Phillies in December, had a MRI earlier this month after he felt discomfort in his left side while playing catch at home in Arkansas. He called Phillies athletic trainer Scott Sheridan to inform him, and Lee later flew to Philadelphia for an examination by team physician Michael Ciccotti.
The Phillies said the MRI revealed a minor strain near his left latissimus dorsi.
"I didn't throw for a few days," Lee said Monday morning at Bright House Field. "I built back up and here I am. I'm perfectly fine and right with everyone else. It's not really a major issue."
General manager Ruben Amaro Jr. said Lee stopped throwing for eight days. His first bullpen session last Tuesday was noticeably shorter than those of his peers, but Lee has participated in every baseball activity since camp opened.
Lee said he feels no discomfort and expects to be ready to start the season on time.
"We still have plenty of time and I feel fine," Lee said. "It's a non-issue. It was very minor. It's not that big a deal. It's really nothing."
Lee opened last season on the disabled list because of a lower right abdominal strain. He missed a couple starts in September after receiving a cortisone injection in his back. Amaro said Lee might miss a start if the Phillies were in the midst of their season.
"We might be cautious," he said. "He's right on schedule right now. It's pretty much a non-issue for us."
Todd Zolecki is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.