Lilly, on the disabled list since May 28 with left shoulder inflammation, threw three simulated innings in the bullpen, 20 pitches per inning and a rest in between to simulate the normal up and down of game conditions.
"I really let it go, so we'll see how it is tomorrow," Lilly said.
Manager Don Mattingly said if Lilly comes out of the session with no discomfort, he would likely follow with another bullpen session, then a simulated game against hitters or a Minor League rehab assignment.
"We're still at a point where we have to see how he is tomorrow; that determines what's next," Mattingly said. "Either he gets toward a sim game or with a bullpen in between, moving forward. But we don't start the clock [on a rehab assignment] until he's ready to start it."
Right behind Lilly is reliever Matt Guerrier, who threw half-speed off a mound Monday and has his first bullpen session planned for Wednesday.
"This is a real test," said Guerrier, who has been on the DL since April 19 with right elbow tendinitis. "I think the six weeks of rest has helped. Now I have to throw and not think about it. But it feels a lot better."