The Philadelphia 76ers fear All-Star center Andrew Bynum might have done additional damage to his knees while bowling, according to sources close to the situation.

Multiple sources told ESPN on Saturday that Bynum suffered an unspecified injury this month while bowling. On Friday, Bynum revealed that -- on top of the issues with his right knee that could keep him sidelined until January -- he also had suffered a "setback" with his left knee.

"I had a little bit of a setback, and we're just working through some issues with the right knee," Bynum said before the Sixers beat the Utah Jazz on Friday night. "I kind of have a mirror thing going on with my left knee. I don't know what's going on, but the doctors are saying pretty much that it's a weakened cartilage state."

There are several activities that are prohibited in standard NBA player contracts, but bowling is not one of them. Bynum is known to enjoy bowling.

Multiple calls to Bynum's agent, David Lee, were not returned.

Bynum, yet to play or practice with the team, was acquired in the blockbuster four-team trade in August that also sent Dwight Howard from the Orlando Magic to the Los Angeles Lakers.

In September, Bynum went to Germany to receive Orthokine therapy injections in both knees. In October, he received preventative lubricating injections. The reason those procedures were done so close to the season, the team said, was to maximize their effects. At some point, however, Bynum developed a bone bruise that has sidetracked his rehabilitation and recovery.

The Sixers have pushed back Bynum's hoped return date three different times and have released a series of detailed statements on the matter. But before Bynum's comments, there never was any mention of left knee pain or weakened cartilage.

"I've just got to wait for the cartilage to get stronger, and that's pretty much what's going on," Bynum said. "The pain is about the same, but there is swelling in both knees that we have under control. It's the same spot, bone bruise on both sides."