OK, it helps when his defense scores a huge momentum-turning touchdown at the end of the first half — and then hangs on to withstand a final, furious threat at the end of the second half, surviving only when Dallas’s 12th penalty of the game negated what appeared to be the winning score.

DeAngelo Hall returned a fumble 32 yards for Washington’s only touchdown, and McNabb completed 15 of 32 passes for 171 yards in his Redskins debut last night in a season-opening 13-7 win over the Cowboys.

It was also the Redskins’ curtain-raiser for Mike Shanahan, and the longtime Denver Broncos coach watched his defense keep the Cowboys at bay when it mattered.

The Cowboys (0-1) moved inside the Redskins’ 40 on three of their first four drives and didn’t score a point, but the drive everyone will remember is the last one, which came after Graham Gano made a 49-yard field with 1:50 remaining to give the Redskins a 13-7 lead.

After the kickoff, the Cowboys started at their 19 with 1:45 left. They converted a fourth-and-10 at Washington’s 43 when Tony Romo hit wide-open Miles Austin over the middle for a 30-yard gain with 12 seconds to go. Three plays later, Romo had only three seconds left on the clock. He scrambled to find Roy Williams open in the end zone.

But, while Williams and the Cowboys were celebrating, the official was calling Alex Barron for holding Redskins linebacker Brian Orakpo, negating the score and providing a fitting end to a game in which the Cowboys made mistake after mistake. Barron was starting because Marc Colombo missed the game with a knee injury.

McNabb was playing the Cowboys for the third straight game, the first player to do so since the 1970 merger. His Eagles lost to Dallas, 24-0, to end regular season in 2009, then 34-14 the following week in playoffs in his last game with Philadelphia before the Easter Sunday trade that sent him to the Redskins. He now trails Romo, 5-3, when the two quarterbacks go head-to-head.

Hall gave Washington a 10-0 lead on the final play of the second quarter when he forced a fumble by Cowboys running back Tashard Choice on a screen pass. The cornerback scooped up the ball, running down the left sideline and somersaulting into the end zone. It was Hall’s third career touchdown.

The Redskins drove 69 yards on their opening possession, with McNabb completing 3 of 5 passes for 32 yards and adding a scramble for 17 yards. Gano ended the possession with a 29-yard field goal.

The Cowboys had a chance to tie the game early in the second quarter, but David Buehler’s 34-yard field goal attempt was wide right. The kick was Buehler’s first NFL field goal attempt.