Allen was forced into a starting spot just four games into his rookie season when Mark Tuinei suffered back spasms on the road against the Washington Redskins. He received the game ball for helping keep the Redskins sackless for the game. He recorded his first NFL start at left tackle, replacing an injured Tuinei against the Arizona Cardinals. This marked the first time a rookie offensive lineman had started for Dallas since November 24, 1991 when Erik Williams started at right tackle against the Redskins. Allen returned to a back-up role for the next two weeks, but early in the morning on October 24, Williams was injured in an automobile accident and was out for the rest of the season, after which Allen was moved into the starting line-up at Williams' right tackle position permanently, beginning with a game at Cincinnati on October 30. He received the game ball, along with Tuinei, for helping hold the New Orleans Saints without a sack in a Monday night win in New Orleans on December 19. In that Monday night contest, the 325-lb. Allen shocked the Saints by running down linebacker Darion Conner from behind on an interception return early in the game, when Allen was running from a standing start at the moment of interception. The game's announcers talked more about Allen's amazing feat than the actual interception. In his first playoffs, Allen received the game ball against the Green Bay Packers on January 8, when the Dallas offense recorded 450 total yards and Troy Aikman completed 23-of-30 passes for 337 yards. Allen sprained his left ankle during the game, but he returned to play. In the NFC Championship Game at San Francisco, he struggled playing through three quarters on his injured ankle before leaving the game.