Starting offensive tackle Marc Colombo left Sunday's afternoon practice in a cart with a right knee injury apparently suffered at some point during practice, and will have an MRI for further examination.
Colombo missed seven games last year with a fractured fibula and a high ankle sprain suffered Nov. 15 against Green Bay. He returned in time for the playoffs and entered training camp healthy, too.
Rookie Sam Young replaced Colombo at right tackle. Robert Brewster has worked there, but he has moved to the left side while swing tackle Alex Barron recovers from a sprained ankle.
If Colombo must miss any time, the Cowboys will be without two of their top three tackles. Barron is rehabbing but his status is uncertain for the Aug. 21 game at San Diego.