"At first, I wasn't feeling anything and was feeling good about that," he said, "but near the end, I felt it come on a little bit. It is not super painful, but it doesn't feel good, either."
Lee returned to the clubhouse and resumed treatment.
The Mariners are preparing to start the regular season in Oakland on April 5 without the former American League Cy Young winner, acquired from the Phillies for three players in December.
A rotation with right-hander Felix Hernandez and Lee in the first two spots represented a potential return to playoff contention for the franchise.
But a foot injury that required surgery got Spring Training off on the wrong foot for Lee, and an incident near home plate in a Cactus League game against the D-backs in Tucson on March 15 caused the latest injury.
While backing up home plate, Lee had what appeared to be a minor collision with Chris Snyder, the on-deck hitter. Lee remained in the game and pitched into the third inning before being ejected after his final pitch of the day sailed over Snyder's head.
Lee said later that he "felt something" in his right abdominal area that day, but it didn't bother him that much until he threw his regular bullpen session on Thursday in Peoria.
"That's when I really started feeling it," he said. "It was worse [than in the game]."
Lee returned to Seattle for an examination, which disclosed the strain.
"I really don't know what the plan is right now," he said. "I am just going to ease back into it and hopefully things fall back into place. Only time will tell how long that will take."
"He'll play catch every other day and we'll do that two or three times and see where we're at," head athletic trainer Rick Griffin said.
Manager Don Wakamatsu said he was pleased that Lee "did not feel any sharp pain in there" during Tuesday's session. "We won't know much more until [Wednesday] and see how he reacts."Even so, he will not be rushed back into action.
"Caution has to come first," Wakamatsu said. "You're looking at a long season and we have to make sure we don't rush him back."
Jim Street is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.