General manager Danny Ainge made the announcement during practice Saturday and said "we don't know when he'll be ready." Game 2 of the best-of-seven Eastern Conference series is in Boston on Tuesday night
O'Neal received therapy before the practice then ran briefly before stopping because of the pain.
He had missed 27 games with a sore right leg then returned on April 4 against Detroit. But he lasted just 5 minutes, 29 seconds, scoring six points, before leaving again with the calf problem and has sat out the last six games. Team physician Dr. Brian McKeon said the injuries are related.
"He's actually made progression every day," McKeon said, "but this is a tough injury. The blood supply in this area is so weak and so poor and (it takes) time. ... It's just hard to get enough healing potential in that area."
He said surgery was not an option and that O'Neal would try running again when his leg strength reaches a certain level and he feels comfortable enough to do it.
"He may come back in three days and feel like it's better" and try to run again, Ainge said. "It's not like this magic test. The test is, `Does it hurt you? Run up and down the court and does it hurt?' That's the test.
"We're just day to day."
O'Neal played 37 games this season, averaging 20.3 minutes, 9.2 points and 4.8 rebounds. The Celtics signed him and Jermaine O'Neal to two-year contracts in the offseason to play center.
Jermaine O'Neal was limited to 24 games by a knee problem that required surgery but has played in six of the last eight, including 36 minutes, 54 seconds in the next to last game of the season, a 95-94 overtime loss at Washington last Monday.
"If he doesn't come back (and) Shaq, then we would have some problems," Celtics coach Doc Rivers said. "Having J.O. back gives us a luxury and that's why Danny went out and signed two guys so we'd have one, and right now I'll take it."