O'Neal said after Friday's game that he misread the message on the grease board in Boston's locker room and didn't realize the team was practicing before it's flight.
"It was a miscommunication," he said. "The only thing I saw on the board was flight at 3. That's all. I never miss practice."
"I apologized to the Big Three but not anyone else," O'Neal said, according to the Boston Globe. "I just apologized to the generals whom I'm consulting. It's not a big thing. It's not like I'm one of those players who said [forget] it, I'm not coming to practice today. But it won't happen again."
Celtics coach Doc Rivers gave his players the weekend off, but O'Neal's mix-up gave him a third day of rest, which Rivers joked wasn't the end of the world for a 38-year-old veteran with no history of skipping sessions.
"He got the times mixed up," Rivers said Monday. "He missed one, so he owes us one. We're good."
Information from The Associated Press was used in this report.