The Miami Heat often talk about having a “next man up” mentality.  They may have to utilize it in their Eastern Conference first-round series against the Atlanta Hawks. Guard Kyle Lowry sustained a hamstring injury in Friday’s Game 3 and is questionable for Game 4 Sunday in Atlanta.  “All I know right now is it’s a hamstring,” Heat coach Erik Spoelstra said after the Game 3 loss. “But I do not know the severity of it and we’ll find out more (Saturday).” Lowry watched from the bench as the Heat blew a fourth-quarter lead and lost on Trae Young’s floater with 4.4 seconds left. On the next possession, Jimmy Butler…