CHICAGO -- As Dontrelle Willis said, his knee injury could've been worse. But the diagnosis wasn't good enough to keep him off the disabled list.

An MRI exam Saturday confirmed a hyperextended right knee, along with minor stretching in the back of the knee. With Willis certain to miss at least one start, and the Tigers unable to skip his turn in the rotation, they placed the left-hander on the 15-day disabled list.

Both are firsts for Willis in his five-year Major League career. He has not missed any of his 164 starting assignments since the Marlins called him up in 2003.

"You know he's going to miss a start, and we don't want to take any chances with it," team president/general manager Dave Dombrowski said. "Really, at this point, we can't. It is a hyperextension, so hopefully in the 15 days, he'll be ready. But we'll see.

"He asked us to wait [to put him on the DL] and see what happens, but you could tell he's walking gingerly, and we just don't want to take a chance. You can't take a chance on that, because his arm is healthy. I've seen many guys in their time [who] when their knee's bothering them a little bit, change their delivery a little bit and hurt their arm."

Willis' right leg gave way on his first pitch to Orlando Cabrera, the second batter he faced. He continued for a few pitches, walking Cabrera and falling behind on Jim Thome, but head athletic trainer Kevin Rand noticed the knee clearly affecting his delivery. Manager Jim Leyland then decided to pull Willis from the game.

Willis sounded an optimistic tone after the game, saying his knee was more stiff than painful. Even with his record of durability at an end, he admitted it could've been a lot worse.

"There's no tear, so that's good," Willis said.

Rand said the MRI also revealed stretching of the posterior knee capsule as well as the hamstring tendon where it attaches to the back of the knee. Because it's his landing leg, Rand added, the Tigers will be extra cautious in his recovery to avoid aggravating it.

Assuming no rainouts, Willis' spot in the rotation next comes up Wednesday in the opener of a two-game series at Cleveland. At that point, they'll recall right-hander Armando Galarraga from Triple-A Toledo to take his spot for what likely will be two starts. Detroit doesn't have an off-day scheduled until April 28, two weeks from Monday.

Galarraga, acquired from Texas in a Minor League deal in February, was the recommendation of club officials in Toledo after the right-hander won both of his starts in the Mud Hens rotation. The 26-year-old Venezuelan allowed three runs on seven hits over 12 innings with 11 strikeouts.

"He's thrown the ball very well for us down there," Dombrowski said. "After we sent him out in the spring, he continued to throw the ball very well down in Triple-A for us. [Virgil] Vasquez has pitched well, too, but we think Galarraga's the guy that deserves it at this point. He's the guy that they recommended."