The team announced Thursday that he was diagnosed with a Grade 1-Plus hamstring strain and he will be re-evaluated before the next road trip, which begins with a Friday game against the Brooklyn Nets.
It now appears he could miss even more time.
It's been a common injury for Harden over the past two seasons. The six-time All-Star missed seven games during the 2017-18 season with a strained hamstring. His absence didn't prevent the Rockets from leading the Western Conference with 65 wins.
Things have been more difficult for Houston to start the 2018-19 campaign. The reigning Southwest Division champions stumbled out of the gate with three losses in their first four games.
Losing Harden for any length of time creates a huge void in the Rockets' lineup. He leads the team in scoring (28.5 points per game) and assists (9.0). Eric Gordon will take over as a starter for head coach Mike D'Antoni.