O'Neal is struggling to be fit to face the New York Knicks on Friday.
"He is going to try to do some things (Thursday) and we'll see," Rivers said. "He's had some improvement, but I don't know how much," coach Doc Rivers was quoted as saying on NBA.com.
The same calf injury also ruled O'Neal out of Boston's last six regular-season games. The veteran center had previously missed 27 straight games with a sore right leg before sustaining the troublesome calf injury in his first game back.
However, Rivers expects O'Neal to play a part in Game 4 in New York this Sunday.
"If he goes on the trip that means we think he'll play that Sunday," Rivers said.
The Celtics won the first two games of the series in Boston with Jermaine O'Neal starting both games at center.
