NORMAN, Okla. (AP) Oklahoma defensive lineman Charles Walker has chosen to leave the program and enter the NFL draft.

Defensive coordinator Mike Stoops made the announcement after Tuesday's practice.

Walker, a second-team All-Big 12 pick by the coaches last season, missed the national semifinal last season against Clemson with a concussion. This season, Walker played in only four games, also because of a concussion. As of Monday, his playing status was uncertain, and he was to meet with trainers.

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Walker is the second defensive lineman the Sooners lost for good this week. Coach Bob Stoops announced Monday that Matt Dimon and the program have agreed to part ways. Dimon has suffered from a strained Achilles and hasn't played since the Ohio State game in September.