Willis had not made it to the ballpark when Tigers manager Jim Leyland had his pregame meeting with the media and revealed the switch.
Thomas has pitched in four games in relief this season, allowing six runs in 7 2/3 innings. He has two strikeouts and seven walks. It will be his sixth career Major League start. Thomas is 3-2 with a 4.38 ERA as a starter.
Leyland indicated he might have to go early to a bullpen that has already worked 56 innings in 17 games.
"I don't know how much Thomas can throw," Leyland said. "It depends on him throwing strikes."
Leyland said he considered turning to right-hander Eddie Bonine for the start, but he doesn't want to jeopardize Bonine's momentum to start the season. Bonine, who features a knuckleball, has a 1-0 record and has allowed two runs in nine innings.
"I want to be careful with Bonine," Leyland said.
Todd Wills is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.