James Harden experienced what the Brooklyn Nets are calling a "setback" in his hamstring rehabilitation and is now out indefinitely.

"Back to square one," Nets coach Steve Nash said Tuesday. "We'll rehabilitate him and get him back whenever we can, and who knows when that will be. We'll support James and our performance team in getting him back in as best condition as possible. And hopefully that comes sooner than later, but there's no guarantee."

Harden experienced the setback Monday during an "on-court rehab session" and had an MRI on Tuesday, according to the team's statement. Nash said that Harden didn't fall or execute a specific maneuver that caused him discomfort.

"He just felt it," Nash said. "He didn't fall or stumble or anything out of the ordinary; he just felt something maybe in the ballpark of a strain. Then the scan revealed he did suffer a setback. So not much more to it other than just disappointment and that we have to rebuild and get him going again."

Harden has missed the Nets' past six games. He last played on April 5 against the New York Knicks, when he logged just four minutes before suffering a strain. Harden had been dealing with some hamstring tightness before exacerbating the issue against the Knicks. He had been progressing well in his rehab and traveled with the team on its current road trip to continue to ramp up his 3-on-3 and 4-on-4 workouts.

On Friday, Nets general manager Sean Marks said Harden had been "progressing well" and that he had "met the markers up to this point." Now, the Nets will be forced to continue on short-handed for the foreseeable future.

"He'll be back when he's back," Nash said. "That may not be until the playoffs. It may be sooner. I don't know."

Kevin Durant, who is also out for Brooklyn with a left thigh contusion, has a "more positive" prognosis than Harden, Nash said. Durant was diagnosed with the contusion after getting tangled up with Miami Heat guard Trevor Ariza on Sunday. Nash said Durant is day to day.

Harden is averaging 25.2 points, 8.0 rebounds and 10.9 assists per game for the Nets. Durant is averaging 27.3 points, 6.7 rebounds and 5.2 assists per game.