Not only has he never been on the disabled list since breaking into the big leagues in 2003, he hasn't missed a scheduled start. He'll miss at least a couple of those with the 15 days' absence.
But it's not just the missing starts that can be a test. It's working out while the team is preparing to play, or dealing with the slow progress when he is working out or taking the extra days to make sure he's all right when he does start feeling fine.
He isn't really in pain, but his knee is stiff.
"I don't like it. I don't like it at all," he said. "I don't like being hurt. Nobody does, but I've never been on the DL before, so it's all new to me. I got hurt two or three days ago, and I'm trying to get back on the field right now."
That isn't going to happen. Though there's no firm timetable, manager Jim Leyland reiterated head athletic Kevin Rand's belief that the team has to be cautious with Willis, because the hyperextended right knee is part of his plant leg. And that was clearly expressed to Willis.
Willis isn't naturally patient, but he has an appreciation of their standpoint.
"When you have an injury, especially a lower-half injury, you don't want to do something to try to overcompensate with your upper half," Willis said. "I think that's what the biggest concern is. They don't want me to put more strain on my arm, and then my arm blows out as opposed to my leg.
"You do the best you can, because you know that their best interest is exactly what your interest is. They want you to be on the field, and I understand. But I'm always used to playing if I feel like I can go out there."
This won't be much of anything he's used to experiencing. His only DL stint came at the tail end of the 2002 Minor League season, when he had a left shoulder strain, so he didn't have a season to try to get back to that fall.
For now, he's going to try to be a cheerleader in the dugout, but even that can be difficult for him. He'll join the club on the road later this week in Cleveland and Toronto, but when his teammates take the field, he'll have work to do.
"I'm almost like a kid at the playground," he said. "Everybody else is having fun. I want to shoot the ball, too."
Jason Beck is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.