O'Neal didn't travel with the team for Saturday night's game at Washington. The 38-year-old center left in the first quarter of Friday night's home win over Utah with a sore right hip.
Rivers said if the injury were something to be concerned about, he probably would have been told. The coach said he's "just assuming that it's nothing that bad."
Rivers said he'll find out more when the team resumes practice Monday. The Celtics' next game is home against Cleveland on Tuesday.
O'Neal missed his 10th game of the season Saturday night. He's previously been sidelined with knee and calf injuries.
Rookie Semih Erden started in O'Neal place.