The first couple of weeks of Jermaine O'Neal's time in Celtic green haven't exactly gone as planned, with the C's new center bouncing in and out of the lineup with minor injuries, failing to find a consistent rhythm. But now it appears the O'Neal problem is even worse than the C's initially thought.

O'Neal, who sat out the second half of the Celtics' loss to Dallas last Monday and hasn't played since, is now expected to miss two to three weeks due to a sore left knee. The team will now be without one of its co-starting centers until early December.

"That's only a guesstimate," head coach Doc Rivers said of the O'Neal timetable. "We don't know exactly. We're still going through a couple more evaluations."

O'Neal has already missed three of the Celtics' first 10 games — one against the Knicks on Oct. 29, and two this week as the C's finished out their road trip with wins in Miami and Memphis without him.

The team is weighing its options as far as long-term plans to get O'Neal healthy. Rivers sounded optimistic that surgery wouldn't be necessary with the 32-year-old big man.

"I haven't heard that word," the coach said. "So I would say no."

In J.O.'s absence, the Celtics will turn to their other O'Neal to handle the starting center duties. Shaquille O'Neal made his return Thursday night from a bruised knee and played well this weekend, averaging 11.5 points and 6.5 rebounds over his first two games back.

"Shaq's doing good," Rivers said. "He played 22, 23 minutes for us on Saturday, and they were big minutes. He played phenomenal. But again, we have to be careful with him in all our practices."

The other missing Celtic at practice on Monday was Marquis Daniels, but Rivers clarified that health wasn't an issue with him.

"Marquis has a family issue," the coach said. "He probably won't be here [Tuesday], and he may not even be back by [Wednesday]."

The C's play the Washington Wizards at the TD Garden on Wednesday night. Their active roster for that matchup is still very much up in the air.