By Andrew Holleran The Boston Celtics can’t afford to lose Marcus Smart against the Milwaukee Bucks. Smart, the NBA’s first Defensive Player of the Year at the guard position since Gary Payton, left Game 1 of the second round series against Milwaukee with an apparent injury. The former Oklahoma State star was holding his shoulder as he ran into the locker room with a trainer. Smart appeared to be in serious pain. Thankfully, Smart is expected to return to Game 1 in the second half on Sunday afternoon. The injury isn’t believed to be serious. “Celtics say DPOY Marcus Smart is expected to return…