How long will Robert Williams be out? Medical expert explains surgical options.

Celtics big Robert Williams needs surgery to repair the torn meniscus in his left knee, but how long will he be out?

That depends on which surgical option he chooses, as Dr. Jess Flynn — a Boston sports medicine doctor and frequent media analyst — outlined in a post on her website. In some cases, athletes are able to rehab and avoid surgery for the rest of a season, but that isn’t an option for Williams.

“He has some optimism that possibly he can come back and guys have to step up in the meantime,” Ime Udoka said on Monday. “We’ve done it all year and hopefully, he’ll be back sooner than later. We’ll see in the next few days, but that should give him some hope and optimism to heal up and rely…