Lee was economical over six shutout innings in a rehab start Sunday before the Rainiers dropped a 4-1 decision to the Salt Lake Bees.
Expected to throw around 85 pitches, the 2008 American League Cy Young Award winner exited after 68. He allowed three hits, struck out four and did not walk a batter.
"I felt strong the whole time," Lee said. "It's really more of a precautionary-type deal to ease back into it. But physically, I feel like I could keep going, for sure. Still, it's better safe than sorry."
One of Seattle's prized offseason acquisitions, Lee strained his abdomen in Spring Training. He's expected to rejoin the Mariners this week and make his season debut on Friday against Texas.
"I'm anxious. I can't speed up the clock. I've stil got work to do between now and then," he said.
The 31-year-old left-hander retired the first nine batters, then gave up back-to-back singles to Peter Bourjos -- who bunted -- and Freddy Sandoval. But he got Michael Ryan to ground into a double play, retired Terry Evans on a fly ball and pitched a perfect fifth.
In the sixth, Hainley Statia reach on an infield single and was sacrificed to second by Brian Walker. Bourjos lined out and Sandoval flied to left to end Lee's afternoon.
"It's just one of those days where I wasn't able to kind of throw [my curveball] for a strike," he said. "I did throw a couple of good ones down that I got swings and misses with. Other than that, fastball, changeup, cutter was all right there where I want it to be."
The Bees pounced on reliever Mike Koplove (0-1) in the seventh, getting a sacrifice fly from Gary Patchett, a two-run single from Walker and an RBI single from Sandoval.
Trevor Bell (1-0) outlasted Lee to get the win, limiting the Rainiers to one run on six hits over 6 2/3 innings with two walks and three strikeouts.