Howard suffered the injury in the first quarter of Game 1 of the Western Conference finals against the Golden State Warriors on Tuesday night.
Howard, who finished with seven points and 13 rebounds in 26 minutes, isn't sure whether he'll be ready for Game 2 on Thursday.
"It's very painful," Howard said after the Rockets' 110-106 loss. "I tried to play it off as much as possible, but I couldn't give my teammates what I needed to give them."
At the halfway point of the first quarter, Josh Smith crashed into Howard's left leg after missing a shot attempt. Howard stayed face down on the floor for a few seconds and slowly got up, then was looked at by trainers at a timeout with 5:50 to play. He walked down the baseline, hopped up and down and bent both legs.
He then went into the tunnel to loosen up the leg after he was replaced by rookie Clint Capela. Howard returned to the bench area with about three minutes to play in the first quarter.
Howard did start the second half and played nearly nine minutes, going 0-for-2 from the field. However, it was clear he wasn't himself; he didn't have much lift and seemed to move around gingerly.
Coach Kevin McHale took Howard out of the game less than a minute into the fourth quarter for Capela again, and the center didn't return.
Howard dealt with right knee issues during the course of the regular season and was absent for nearly 2½ months with swelling in his right knee. In total, Howard missed 41 games, returning late in the regular season.
In the postseason, Howard has been a force, averaging 17.2 points and a playoff-leading 13.8 rebounds per game.
"Dwight wasn't able to play, but we played over a half a season without Dwight," McHale said. "There's no excuses. We've got to play. There's balls there to be had, there's rebounds there to be had. There's baskets there to be had, there's layups there to be made. We've got to do that. Dwight didn't prohibit us from -- we were 26-for-51 at the rim. We got their setting and we didn't finish. That team, [Andrew] Bogut barely played, so there's no shot blocking. You've just got to go in there and finish it."