Turns out it was hyperextended -- that's bad enough -- after he slipped in front of the mound while delivering a first-inning pitch Friday night.
With Willis sticking around just long enough to throw 14 pitches, the Tigers regrouped behind a solid bullpen to beat the Chicago White Sox 5-2 for their second win in 10 games this season.
"I thought initially I blew it out, but that's not the case," Willis said. "I'm able to put pressure on it now. As long as they give me a lot of Advil, I can go back out there."
Willis will be re-evaluated and when he might be able to pitch again was unclear.
The lefty walked leadoff hitter Carlos Quentin. On his first pitch to Orlando Cabrera, his right knee that he plants while delivering gave way and he slipped in front of the mound. The mound may have been slick because the start of the game had been delayed 6 minutes by rain on a 46-degree night.
A trainer and manager Jim Leyland came out, but Willis told them he was OK to continue.
After Willis walked Cabrera and threw a wild pitch with a 1-1 count on Jim Thome, the trainer and Leyland went back to the mound. Willis again appeared to tell them he was OK, but Leyland removed the lefty and brought in Aquilino Lopez.
"I was just driving on the mound and the cleat missed and I hyperextended my knee. I feel positive. It just feels stiff right now," Willis said.
"After it happened, I tried to continue to throw but my leg just felt numb all the way up. ... I think I just missed [the plant]. I got a little long and caught the back of my cleat."
Magglio Ordonez, who had just one RBI entering the game, hit a two-run single in the third to put the Tigers ahead 3-2. Brandon Inge also had an RBI single in the inning that started with Edgar Renteria's double off Jose Contreras.
"I've seen things like that, unfortunate things have happened and guys have come together and tried to pick up and they did a wonderful job of that," Willis said.
Detroit's had a rough start with injuries and some poor play. And then lousy weather made for a long trip in from Boston early Friday.
"We get in 3, 4 o'clock in the morning. Terrible weather. We're going tough," Leyland said. "Things haven't been going good and we come out and get an effort like that. ... That can put to rest anybody that thinks this club doesn't bust every day."
Contreras (0-1) allowed six hits and four runs in 6 2/3 innings.
Thome was ejected for just the third time in his career in the fifth inning after he was called out on a 3-2 pitch by plate umpire James Hoye. An angry Thome was separated from Hoye by manager Ozzie Guillen.
"You eject Jim Thome, a guy who never says anything to anybody," said Guillen. "He said he thought the ball was low and the umpire saw the ball different."
The White Sox had first and second at the time against Grilli and after Thome struck out, Konerko walked to load the bases. But Jermaine Dye hit into force out when Renteria made a nice play on his grounder and threw to second.
Willis, acquired from Florida along with Cabrera in the offseason, managed just four strikes in 14 pitches in his second start with the Tigers. In his first outing against Chicago, he walked seven in five innings ... The Tigers have been hit hard by injuries and are already without leadoff hitter Curtis Granderson (broken hand) and reliever Fernando Rodney (sore shoulder). DH Gary Sheffield was out of the lineup Friday with a sore finger. He tore a tendon in the finger last week. ... White Sox CF Nick Swisher was out of the starting lineup to rest a hip pointer but replaced Thome in the DH spot. ... The White Sox unveiled a monument to their 2005 World Series winner before the game in a plaza in front of U.S. Cellular Field.