Walker started four games for the Sooners this season before suffering a concussion Oct. 1 against TCU. He has not played since.
"Quitting on your teammates is hard to take as a coach," Oklahoma defensive coordinator Mike Stoops told reporters after Tuesday's practice.
Walker was a second-team All-Big 12 selection last season after racking up 36 tackles, including 10 for loss, and six sacks. However, he missed the Orange Bowl against Clemson, due also to a concussion.
ESPN draft analyst Mel Kiper Jr. recently ranked Walker the No. 6 defensive tackle eligible for the upcoming draft.
Oklahoma has two games remaining in the regular season, against West Virginia on Saturday and Oklahoma State on Dec. 3. The Sooners are 8-2 and first in the Big 12 standings.