Jermaine O'Neal may be forced to undergo knee surgery.

O'Neal has missed 21 games this season because of lingering soreness in his left knee. He underwent an MRI on Wednesday and met with the front office on Thursday.

"[In-season surgery] was definitely something we talked about the first time I was out for the extended period of time," O'Neal told WEEI (850 AM in Boston) on Thursday during a weekly Celtics call-in segment.

"We wanted to try a couple of other options. We may be looking at that situation now. [Surgery] is something that I will eventually need, at some point -- at the end of the season or if it's in-season -- but you want to be around. You don't want to miss extended periods of time ... I already did that. So you make your decisions as a player and you listen to the staff, and, if they have a certain way, then you try that out and if it doesn't work you have to go with the next-best scenario.

"I'm going to run in to see the doctor again in just a second with the team trainers and [Celtics president of basketball operations] Danny [Ainge] and those guys and we're going to make a decision on ultimately what's best for me to get better and be done with the ailment and be back on the court full time."

Celtics coach Doc Rivers hinted that in-season surgery was the likely option after a two-month rest didn't work.