The left-hander threw a 25-pitch bullpen session Friday, his latest step toward joining the rotation, but the Mariners don't expect Lee to be removed from the 15-day disabled list before the end of April.
"Essentially, we're looking at the first week in May," manager Don Wakamatsu said.
The Mariners acquired Lee from the Philadelphia Phillies in a four-team trade on Dec. 15, 2009.
The next step for the 2008 AL Cy Young Award winner will be a 45-pitch bullpen session Sunday, followed by a 55-pitch simulated game a week from Friday. After that, Lee would go on a minor league rehab assignment.
Asked how he fared in the bullpen Friday, Lee said "normal, felt good" as he exited the field before Seattle's game against the Texas Rangers.
Lee threw off a mound in Oakland on Wednesday for the first time since he was injured on March 18. He was originally slated to throw on flat ground this week but felt healthy enough to step on the mound Wednesday and Friday.
Wakamatsu said Lee is eager to increase his workload, but the Mariners want to avoid any setbacks with an injury that normally takes as long as six weeks to heal.
"He feels like he can go, but the last thing we want to do is set him back," Wakamatsu said. "So we're trying to juggle it so that we can accomplish both, to get him back and we don't have the setback."
Erik Bedard, another injured left-handed starter, played long toss on Friday and will take 45 more long-toss throws on Sunday.
Bedard is on the 15-day DL recovering from left shoulder surgery and isn't expected to be activated by the Mariners until July or August.