"He feels great," Rivers said prior to the Clippers' game against the Orlando Magic at Staples Center. "He told me that he's felt as good as he's felt in a long time. I think that the four to five weeks ... it's still that."
Griffin underwent a routine arthroscopic procedure to remove loose bodies from his right knee on Dec. 20.
Griffin will miss his 13th consecutive game on Wednesday night. Rivers wouldn't specify a return date, but team sources said the Clippers are hopeful he could return during the last week of January, which would correspond with the initial diagnosis of four to six weeks.
"I don't know if he's ahead of schedule, on schedule, but all things are go for him -- and that's very positive for us," Rivers said.
Prior to the injury, the five-time All-Star was averaging 21.2 points, 8.8 rebounds and 4.7 assists this season on a true shooting percentage of 54.3 and a Player Efficiency Rating of 22.32.
The Clippers under Rivers have typically exercised caution with regard to timetables. Guard Chris Paul recently sat out sevens game over a two-week span while nursing a strained left hamstring. Rivers said repeatedly that Paul could have played if the Clippers were in the playoffs but that the longer regular season demanded discretion.