The Chiefs said Johnson sustained a mid-foot sprain when he was tackled from behind after a short gain in the fourth quarter of Sunday's 33-22 loss to Green Bay. He was helped off the field by two trainers and was unable to put any weight on the foot.

The fifth-year back, who set an NFL record last season with 416 carries, has been off to a slow start after missing camp before signing a six-year, $45 million contract.

His absence could be a blow to what is already one of the league's feeblest offenses. It would also elevate Priest Holmes, the former Pro Bowler who has resurrected his career after missing almost two full seasons with a spinal injury.

"They're running some more tests," coach Herm Edwards said late this afternoon. "(Johnson) came in to get treatment, but they took an MRI and they're still studying it. We won't know for sure until probably (Tuesday) sometime exactly what the severity is and if he's going to be out, and if so, how long."

Holmes had four touches yesterday for 16 yards as the Chiefs (4-4) remained tied with San Diego in the AFC West. The 34-year-old former Pro Bowler has had only seven carries for 17 yards since he returned three weeks ago, capping a remarkable and entirely unexpected comeback from a career-threatening spinal injury in October 2005.

Holmes would probably split duty with rookie Kolby Smith, who was taken in the fifth round out of Louisville, but has barely gotten on the field.