The senior also missed spring practices and April's spring game with a shoulder injury.
During Walker's sophomore season, in 2010, he played the first four games before suffering a knee injury and missing the next six contests. He returned for OU's final three games that year.
For his career, Walker has recorded 24 total tackles, one sack and two forced fumbles. He was slated to start at defensive tackle before his departure from the team.
OU beat UTEP, 24-7, without Walker, but the Sooner defense struggled against the Miners' sophomore running back, Nathan Jefferey, who rushed for 177 yards.
Seniors David King and Jamarkus McFarland played virtually the entire game because of OU's lack of depth at defensive tackle. Redshirt freshmen Jordan Phillips and Marquis Anderson played late Saturday, when the win was essentially clinched, but coaches did not put the backups in for any meaningful snaps.
Walker's news came just a couple weeks after another senior defensive tackle, Stacy McGee, was suspended indefinitely for violating university policy. McGee has 14 career starts.
“We'll be fine,” King said Monday. “Once we get these young guys accustomed to the game, we should be fine depth-wise.
“We should get Casey back here in a few weeks, and whenever coach Stoops lifts Stacy's suspension, we'll be fine.”