"They want to make sure everything's OK," Willis said, "because they don't want me to go out there and hurt myself further and be out for three months."
Willis has spent the past three weeks on the 15-day disabled list after he hurt his knee slipping on the mound April 11 at Chicago. His start for the Mud Hens was his first game since then.
The play in question came in the first at-bat of the game. Willis fielded a ball along the third-base line and fired a throw to first off his left leg. According to reports, he seemed to be favoring the leg as the game went on.
Willis said he didn't hurt his knee on the play.
"I just jumped back, so I made sure it didn't [affect the knee]," Willis said. "That's what happened. I know the type of play. I've made that play many times, and I know what can happen, especially with the conditions. It was wet. It rained earlier in the day, and I was just making sure that it didn't happen again."
Willis rejoined the Tigers on Saturday and threw a full bullpen session at the Metrodome on Sunday morning under the watch of pitching coach Chuck Hernandez. He said his knee felt exactly the same Sunday as it did Friday: sore from time to time.
Earlier, manager Jim Leyland said that Willis was tentatively scheduled to throw another rehab start for Triple-A Toledo on Wednesday if everything was fine. The team announced the decision to hold off on that shortly before game time.
Tweak or no tweak, Willis will stay in Detroit this week and work out the knee instead of the original plan. He's now slated to throw a side session on Friday or Saturday before he and the club decide his next step.
"That's the conclusion they came to," Willis said. "It's for my safety. But I'm going to try to get myself ready to get back on the mound, regardless of a timetable. Whether it's four more days or a week or whatever, I'll try to prepare myself."
At the least, this setback means more time for Armando Galarraga, who has stepped into the rotation in Willis' place. Galarraga suffered his first Major League loss on Friday, allowing two earned runs and two unearned tallies over six innings. Despite the tough loss, the right-hander has been surprisingly effective in his first extended big league stint.
Jason Beck is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.