It was a sudden decision on Monday afternoon brought on by a sudden illness. Tigers head athletic trainer Kevin Rand said Willis fell ill on Sunday night after the club returned home from Cleveland, where it finished up a weekend series against the Indians at Progressive Field.
Willis had a 102-degree temperature.
"He didn't sleep very well last night," Rand said. "He was burning up."
Willis was examined by Dr. Michael Workings, the Tigers' physician, to make sure he had nothing worse than the flu. Once Willis was diagnosed, the club asked him to stay home to keep him away from his healthy teammates. So far, no other players have come down with a similar illness, and the team hopes to keep it that way.
It marked the second scheduled start Willis has missed this season for health reasons. The left-hander was scratched from a start last month at Texas with some sort of stomach bug or food poisoning, but he had made his past two starts on turn since then.
Lefty reliever Brad Thomas started in Willis' place on Monday. Thomas filled in for Willis when he was scratched last month, too.
"I had a bunch of pitchers who offered to pitch today," Tigers manager Jim Leyland said. "[Jeremy] Bonderman wanted to pitch, but I'm not going to change everything up. I'm going to just leave it the way it is. We're stuck a little bit, but that's the way it goes."
When Willis missed his start last month, he pitched out of the bullpen the next day, in part to make sure he didn't go too many days in a row without pitching. Leyland said he could have the lefty do so again if he's feeling better, but he doubts that's going to be an option.
"I doubt he'll be well tomorrow," Leyland said. "He has a 102 temperature. I think that's unlikely."
Jason Beck is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.