On the screen is Gerald Wallace, and he is a 6-foot-7-inch terror who leads fast breaks; who tries to dunk everything; who dives after every loose ball as if saving it means saving the world, who hits the floor so often, so hard, with such disregard for his body, everyone calls him “Crash.” “Crash” Wallace. He never thinks about tomorrow. That’s obvious from how he plays, with a level of intensity, effort, and reckless abandon few — if anyone — has matched during his NBA career, now in its 13th season. The Celtics forward only thinks about now. Right now. And right now, Wallace watches that screen from his home, where he studies game tapes of his old self, of what he once could do. And he realizes that he can’t do those things anymore, even if he could not all that long ago. “Five years ago, I was dunking anything and everything that was close to the rim,” Wallace said recently before the team headed to this city, where it will play the Rockets Tuesday at the Toyota Center. “Nowadays, I’ve got to be wide open to dunk.” Wallace is 31. He’s not ancient by NBA standards, with Steve Nash still able-bodied at nearly 40, Ray Allen at 38, Tim Duncan at 37. But Wallace has played 23,338 NBA minutes, and he has played them all hard, so hard that he has endured countless injuries that have exacted a staggering toll. Concussions. A partially collapsed lung. A fractured rib. A separated shoulder. A chipped bone in his wrist. Bruised knees. Sprained ankles. Name a body part, and he has injured it — probably more than once. This season, though, Wallace has realized that he can’t play the way he always has, even if he still wants to. This season, he said he has realized that he can no longer be “Crash” Wallace.