The southpaw, who was acquired by the Tigers this past offseason and given a three-year contract extension worth $29 million, is likely to be activated when rosters expand to 40 players on Monday, but his role with the team will be limited.
Detroit manager Jim Leyland said Willis will throw a simulated game on Tuesday but likely not see any game action until 2009, though he will be available throughout September.
"There's no rush to get him out there," Leyland said. "He will rejoin the team, and we talked about maybe an instruction league and everything, but it's been decided that he'd like to work with [pitching coach] Chuck [Hernandez], which we're all for.
"There's some open mindedness as far as possibly pitching winter ball somewhere. That's not anything etched in stone, that's just Dontrelle open minded to it. But that part hasn't been solidified in any way, shape or form."
Willis was optioned to Class A Lakeland on June 10 after just four starts with the Tigers, in which he issued 21 walks in 11 1/3 innings.
Willis, who won the National League Rookie of the Year Award in 2003, finished second in Cy Young Award voting in 2005 after he put together a 22-10 season with a 2.63 ERA.
The left-hander was promoted to Triple-A Toledo on Aug. 12 and showed some signs of improvement with a better strikeout-to-walk ratio, though the walk totals still were high. He allowed 14 earned runs and 14 walks in 28 1/3 innings.
The priority for Willis right now, though, isn't to have good starts.
"Actually, just having good workouts every day is a priority for me," Willis said. "If you win those battles, then hopefully I can get to that point. [Good starts are] not really a concern of mine. I just want to throw well every time I get on the mound and stay consistent."
Willis also is in much better shape than he was just three months ago, and the injury he suffered to his throwing arm Thursday was nothing serious.
"It was my bicep. There was some cramping and stuff like that, but I'm fine now," said Willis, who had to leave his start after just one inning. "My knee feels great. My knee feels really good. Other than that, just the confidence to be able to do those things and the workouts -- I feel a lot better."
Willis' progress is good news for the Tigers, who spent $29 million on a three-year deal in December just two weeks after acquiring him in an eight-player trade from Florida.
But the money and the investment are less of a concern for Leyland.
"We're doing this for one reason," Leyland said. "People keep talking about the investment we have in him. That's not the issue with the Detroit Tigers. The issue of the Detroit Tigers is we think Dontrelle Willis is a [heck] of a pitcher, and has struggled and has a chance to be a [heck] of a pitcher again.
"We think we got a [heck] of a potential pitcher on our hands. This isn't a guy who hasn't done anything; this guy's done something. We're trying to get Dontrelle prepared to be a successful pitcher in 2009. Is that gonna happen? I can't answer that question."
Leyland isn't forcing the issue of winter ball on Willis, but the manager said he would prefer it if the lefty went.
"Yes, we would like him to go," Leyland said. "But we're not gonna do anything to force that issue. We're gonna make suggestions, and when push comes to shove, it'll be Dontrelle's decision."
David Just is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.