Rivers' back injury was quietly added to San Diego's injury report last week, to go along with the rib injury he has been battling for weeks. By now, Rivers' rib injuries are improved, making his back injury the significant issue, a league source said.
The injury has contributed to Rivers' pedestrian performances the past two weeks, against New England and Denver.
Through the first eight weeks of the season, Rivers completed 68.3 percent of his passes for 2,213 yards, 20 touchdowns and just five interceptions, posting a 109.9 passer rating. The Chargers were 5-3 in those games.
In the past six games, Rivers has completed 66.8 percent of his passes for 1,426 yards, seven touchdowns and eight interceptions, posting an 81.2 passer rating. The Chargers are 3-3 over that stretch.
His back will still be an issue Saturday night, when San Diego plays at San Francisco, but Rivers is expected to play.
He also did not practice Tuesday but said he fully expects to play against the 49ers.
"At this point in the week I think there's no doubt," he said Wednesday.
Rivers was on the field Thursday morning for the Chargers' walk-through practice.
Philip Rivers is back on the practice field this morning. pic.twitter.com/RFg7NVRcut— Eric Williams (@eric_d_williams) December 18, 2014
"Philip had an outstanding day. The ball did not hit the ground today in all the throws he made," coach Mike McCoy said.
ESPN.com's Eric D. Williams and The Associated Press contributed to this report.