He's suffering from a strained or bruised left knee, and was as close to a game time decision as one could be. He came out late for warmups, tested the knee, and gave the go-ahead.
Clint Capela was going to be given the start in his place, and it's not unreasonable to think the Swiss Roll will average a career high in minutes tonight. Every challenge in the playoffs he's been given so far, he's lived up to. This will be his biggest yet, because it's unlikely Dwight plays close to 40 minutes.
Howard had 5 turnovers and just 7 points in Game 1, trying to play hurt. If he's ineffective, Kevin McHale shouldn't push him. But lord, do the Rockets need a fully healthy, fully functional Dwight Howard.