Willis injured the knee while delivering a pitch in the first inning Friday night against the Chicago White Sox and minutes later was taken out of the game.
"We know he's going to miss at least one start and we don't want to take any chances with it," Detroit general manager Dave Dombrowski said after a 7-0 loss to the White Sox on Saturday.
RAYS: Prospect Evan Longoria was recalled from Triple-A Durham and made his much-anticipated major league debut Saturday against the Baltimore Orioles.
Longoria, sent back to the minors despite having an outstanding spring training, replaces third baseman Willy Aybar . The Rays placed Aybar on the 15-day disabled list Friday after he missed his second consecutive game with a sore left hamstring that may sideline him up to four weeks.
Longoria got his first major league hit in the sixth inning. He hit an RBI single off of Baltimore's Daniel Cabrera .
ATHLETICS: Ryan Sweeney left Oakland's game against Cleveland because of a strained right quadriceps muscle.
The outfielder bounced into a double play against Cleveland starter Fausto Carmona in the third inning, then was lifted for pinch-hitter Chris Denorfia in the fourth.
GIANTS: Outfielder Dave Roberts had surgery on his troublesome left knee Saturday, and it could be 12 weeks before he rejoins the team.
Fred Lewis will play in Roberts' spot in left field. Roberts batted .118 with one RBI in six games this season.
Team orthopedist Ken Akizuki performed the 45-minute procedure to clean out cartilage behind his kneecap caused by arthritis and wear and tear. Athletic trainer Dave Groeschner said there was not a tear, as the Giants believed there might be.
The 35-year-old Roberts will use crutches to get around for about 21/2 weeks. He will begin light rehabilitation exercises today to start regaining his range of motion and strength while the swelling from the surgery subsides.
If he progresses well, Roberts could head out on a minor league rehab assignment in eight to 10 weeks.
NATIONALS: Washington made a flurry of moves after losing its ninth consecutive game Saturday, activating closer Chad Cordero from the 15-day DL and optioning starter Jason Bergmann to Triple-A Columbus.
After their 10-2 loss to the Atlanta Braves, the Nationals also said they would rush Wily Mo Pena back from his rehabilitation assignment with Columbus.
Pena had no sooner arrived in Charlotte, N.C., for an expected five- or six-game stint with the Clippers when he got word he was expected in Washington for today's game.