"Willis said he is not depressed now and has not been depressed in the past.
"I'm never depressed at all -- I've always been an amped-up guy," he said. "This is not something where I'm uncontrollable. ... Everybody will tell you in the locker room that I've always been upbeat, regardless of what's going on.
"This is not something where I'm too amped up, I don't know where I'm at, and I'm running sprints up and down the parking lot. This is not something like that. (The doctors) see something in my blood that they don't like. I'm not crazy. My teammates might think I'm crazy. But this is not something like that."
Tigers GM Dave Dombrowski is "fairly optimistic" Willis will return to pitch in the majors this season.