Holdman was injured on Aug. 2 when he was trying to tackle running back Mike Bell, who lowered his left shoulder and struck the 243-pound linebacker in his helmet. Holdman went numb for about 40 seconds before regaining feeling in his extremities.

After being hospitalized overnight, Holdman said he was eager to return to practice to fight for the strongside linebacker job, but he never made it back onto the field.

Coach Mike Shanahan said subsequent tests led doctors to advise Holdman not to risk playing football for a while.

"I don't know for sure exactly what it was. When they go in there and evaluate players they give them a bunch of tests and his recommendation for him not to play or do anything for three months," Shanahan said.

Shanahan said he wasn't sure what roster move he would make in light of Holdman's injury.

"Right now it's hard to tell you exactly what we're going to do," he said. "We are talking to his agents and the doctors. Some of this we just found out."

Holdman, a ninth-year pro in his first season in Denver after spending the past two seasons in Washington, is the second candidate for the Broncos' strongside linebacker job to leave the competition with an injury. Free agent Eddie Moore (knee) was placed on the reserve-injured list after clearing waivers earlier this month.