BOULDER, Colo. -- Colorado might have only three regular tailbacks when it faces Wisconsin in the Alamo Bowl presented by MasterCard next week because of Bobby Purify's lingering ankle injury.

''Bobby is very, very doubtful,'' Colorado coach Gary Barnett said Wednesday.

Purify wears a boot on his right ankle because of the sprain he suffered Nov. 29 during his team's 28-13 victory at Nebraska.

Tailback Chris Brown also missed the team's first practice because of final exams but he is expected to join the team despite a bruised sternum.

Cornerback Phil Jackson, who separated his shoulder during the Big 12 championship game against Oklahoma, said he would play against the Badgers on Dec. 28. Barnett said the injury might not require surgery as originally thought.

Reserve center Derek Stemrich will undergo knee surgery to realign his patella and remove bone chips. Rehabilitation is expected to take four months.

Backup linemen Drew Shader (shoulder) and Clint Werth (knee) also will miss a head start on next year because of surgery.