Harden had 21 points but shot 4-for-19 from the field in his first game since Nov. 2. He said he felt good but was disappointed in his performance.
"I didn't play well, but I've got to be better and I will,'' he said.
Coach Doc Rivers said Harden, who spent just more than eight seasons with the Rockets before being traded to Brooklyn in January 2021, would be on a minutes restriction in his return. Harden didn't seem to be limited much, and played 39 minutes.
But he said having his minutes monitored in this game made it tough to get in a rhythm.
"I was all over the place,'' Harden said. "I'm not used to it. But I'm not making any excuses. I have to do better."
He added that Rivers didn't want him to play as much as he did in overtime.
"I had to fight to stay on the court just because there was a certain plan in place before the game started,'' Harden said. "But once you go out there as a competitor you want to try to win.''
Harden missed 14 games with a tendon strain in his right foot. He missed all eight attempts inside the 3-point arc but still got to the line like he always does and made 9 of 10 free throws. He also contributed 4 rebounds, 7 assists and 2 steals with 7 turnovers.
Porter made two free throws before Harden got Philadelphia within 125-122 with a 3-pointer with about 90 seconds left. But rookie Jabari Smith Jr. added a 3 for Houston to extend the lead to six a few seconds later, and Houston held on for the win.
Harden won the MVP award with the Rockets in 2018 and was a three-time scoring champion in his time in Houston. He received a warm welcome from the crowd when he was announced before the game.
The Associated Press contributed to this report.