Dombrowski said the team has not decided how to fill the void in the starting rotation. Willis' former spot is scheduled for Saturday against the Dodgers.
The roster spot will remain open until reliever Aquilino Lopez returns from the bereavement list on Wednesday.
In Lakeland, Willis will work with the organization's pitching coordinator, Jon Matlack, and Lakeland pitching coach Joe Coleman to iron out severe control problems that have plagued the 2005 Cy Young Award runner-up since Spring Training.
Willis, who signed a three-year, $29 million contract after arriving in a December trade with Florida, has walked 21 batters this season in 11 1/3 innings.
"We knew his walks were up last year, but not anywhere near the extent of this," Dombrowski said. "We studied it very thoroughly. We knew he hadn't pitched as well the last couple years, but we also didn't see this command situation."
Willis had to agree to the move, and Dombrowski said he had a positive attitude toward it.
"He was trying to get back and do whatever he felt was necessary to get him back to being the guy that he can be," Dombrowski said. "He didn't want to hurt the team. He wants to be in a position where he pitches effectively and can help the club."
The move came after a nightmarish outing for Willis on Monday night. He lasted just 1 1/3 innings, giving up eight earned runs on three hits, including two multiple-run home runs, and five walks.
Dombrowski said the team had no plans of sending Willis to the Minors prior to Monday night's start.
"I went into [Monday] night thinking he was going to pitch very well for us," he said.
Willis' first season in Detroit has not gone well. He hyperextended his right knee on April 11, resulting in his first career trip to the disabled list. He made one Minor League rehab start, but aggravated the knee on a fielding play and had to be shut down. He eventually returned to the Tigers, but in the bullpen -- where he made one appearance. He then made a piggyback start last week in Oakland with Armando Galarraga, his injury replacement, and gave up no runs in four innings, setting the stage for Monday night.
"He's not himself right now," reliever Zach Miner said of Willis. "He's proven he's a big league pitcher. He won 22 games a few years ago. As soon as he gets back on track, he's going to be that same guy again. So you pull for him and hopefully he can get there sooner than later."
Miner represents one option to fill Willis' spot in the rotation. He started 16 games for the Tigers in 2006. Another in-house option is left-handed reliever Casey Fossum, who started 25 games for Tampa Bay in 2006 before moving to the bullpen.
The leading Minor League candidate appeared to be right-hander Eddie Bonine at Triple-A Toledo. Bonine has a 9-2 record this season with a 4.48 ERA, but struggled in three of his last four starts.
When asked about filling the rotation spot, Dombrowski said: "We'll tackle that in a few days."
The loss of Willis in the rotation means the Tigers have lost two starters in the last few days. Right-hander Jeremy Bonderman went on the 15-day disabled list last weekend, and could be out for the season after a serious blood clot.
This team's list of issues continues to grow. Manager Jim Leyland reflected on his team's turbulent season after the latest setback.
"It's been a chaotic year to this point. It's just one thing after another," he said. "We haven't had a lot of things go right, obviously.
"I've had more scenarios on my desk this year than the history of my managerial career. ... I really didn't count on all this stuff. But it's not going to change me. I'm not going to get into it; I'm going to grind it out and try to win games, and I hope the players do the same thing. I think they will.
"I think we have plenty here to have a very, very good club and win a lot of baseball games," he continued. "Whether we will or not, I don't know."