Center search resumes after passing on Jamaal Jackson

OXNARD, Calif. -- The Cowboys will continue their search for a center, opting not to sign Jamaal Jackson after his workout Sunday, a source said.

The source said that Jackson, the former Philadelphia Eagles starter who has not played since suffering an arm injury in 2010, was in poor shape.

The Cowboys are in the market for a veteran center because their top three players at the position are injured.

Starter Phil Costa will miss Monday night's preseason opener because of a back strain, but he is expected to return to practice next week.

Bill Nagy and Kevin Kowalski are sidelined by serious ankle injuries. Nagy is likely to need surgery and be out for the season.

Cowboys director of player personnel Stephen Jones said the Cowboys are looking for "insurance" at center, indicating that they are comfortable with Costa as the starter despite his struggles last year.