Source: Tyron Smith, Jurrell Casey gone

USC, which already had depth issues on a thinned-out roster, will be losing two of their top linemen to the NFL draft as offensive tackle Tyron Smith and defensive tackle Jurrell Casey, both first-team All-Pac-10 players, are forgoing their final seasons of eligibility, according to a source.

The 6-foot-5, 285-pound Smith is the fifth-rated underclassmen offensive tackle prospect, according to ESPN draft analyst Mel Kiper.

The 6-1, 305-pound Casey, ranked by Kiper as the No. 4 defensive tackle among underclassmen, was a 2010 All-Pac-10 first teamer who led USC in sacks and tackles for losses and was named USC's Most Valuable Player at their awards banquet Sunday. Casey, a two-year starter, had 4.5 sacks and 11 tackles for losses in 2010 to go along with 67 tackles, two pass deflections, an interception and a fumble recovery.

The news was not all bad for USC as Marc Tyler, the team's leading rusher this season, told ESPNLosAngeles.com that he will return for his senior year.

"I'm coming back," said Tyler, who rushed for a team-high 913 yards and nine touchdowns on just 171 carries as a redshirt junior. "I just want to have that senior year and get the final experience of college."

Bruce Feldman is a senior writer at ESPN The Magazine. ESPNLosAngeles.com's Ramona Shelburne contributed to this report.