Harden made the revelation to ESPN's Lisa Salters after the Rockets' 113-109 Game 4 victory on Sunday afternoon.
Harden went 5-for-16 from the field (including 0-for-7 from 3-point range) and scored 16 points. It was the fewest points Harden has scored this postseason.
Despite the poor effort, the Rockets were able to get support from Nene, who scored a team-high 28 points off the bench and tied an NBA record by making 12 shots without a miss.
"It was pretty tough; we don't make excuses," Harden said in a news conference when asked about his health. "We just try to go out there and get the job done. You build trust, and trust in your teammates all year long. When there's moments like this, guys step up and they did tonight. We have another opportunity in a few days to go out there and win on our home court, and we're going to have to get off to a really good start."
Harden didn't walk with a limp, but he didn't have his usual push on drives to the basket.
Harden, one of the main MVP candidates, finished second in the league in scoring and first in assists in the regular season. He battled through a jammed wrist late in the regular season but didn't miss any games. The only game Harden missed was because of the flu.
"You just always got to step up, you just never know," said forward Eric Gordon, who scored 18 points off the bench. "He had a little bit of a slow start; things are going to happen like that. Guys are going to play through (things) and he probably had an ankle injury, and that's when guys have to keep on stepping up. We're always going to do that for each other. We have the players to do it; we're never going to rely on one person."