Zack Martin ruled out by Cowboys due to concussion

— -- ARLINGTON, Texas -- Just as the Cowboys welcomed All-Pro left tackle Tyron Smith?back from a two-game absence because of a groin strain, they lost All-Pro right guard Zack Martin to a concussion in the second quarter of Thursday's 28-6 loss?to the Chargers.?

Martin was staggered on a run by Rod Smith but remained in the game for the next four plays. During Los Angeles' next drive, he underwent cognitive tests and was replaced by Joe Looney.?

He was ruled out for the game after halftime.?

Martin entered Thursday having played 661 of a possible 673 snaps on the season. He has not missed a game in his career.

Smith missed the losses to the Atlanta Falcons and Philadelphia Eagles because of a groin strain suffered late in the Nov. 5 win against the Kansas City Chiefs. The Cowboys used Chaz Green and Byron Bell in his absence.? Dak Prescott has been sacked 12 times, and the offense has scored one touchdown in the two-game stretch.?

Smith was listed as questionable entering the game after going through three limited practices, which never were at more than walkthrough pace. Smith went through an involved pregame warm-up prior to the Eagles' loss on Sunday, but the plan all along was to have him available to play against the Chargers.

In the first eight games of the season with Smith, Prescott was not sacked more than two times in a game.