Having been eliminated from playoff contention once again, the Buffalo Bills are now forced to dissect what went wrong, especially with a defense that was looking to shine under new coach Rex Ryan.

Mario Williams offered his thoughts this week ahead of a matchup with another eliminated team, the Dallas Cowboys, who will give quarterback Kellen Moore his first NFL start Sunday.

Buffalo (6-8) is set to miss the postseason for a 16th consecutive year, the NFL's longest drought and tied for the eighth-longest in league history.

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"I've been in this game a long time. I understand the fans' frustration," Ryan said. "I feel personally responsible and I don't want to let our fans down. I don't, more than anything. It just pains me that we couldn't deliver this year."

The Bills own the top-rated rushing offense this season but their defense currently ranks 21st with its average of 363.9 yards allowed. Buffalo is also near the bottom with 20 sacks, and Williams has just four after posting a career-high 14 1/2 in his third season in Buffalo.

The defensive end said Ryan's scheme might not have the kind of players it needs just yet.

"There's nobody saying anything that what (Ryan's) doing is wrong. But for what's out there, the personnel might not be the right personnel," Williams said. "What we're doing as a defense just has not worked for us this year. Now I'm not saying about the scheme (like), 'Throw it out. It's garbage or whatever.' No. I'm just saying what is what. That's it."

The Cowboys' woes have mainly been on the other side of the ball as they've failed to overcome Tony Romo's absence due to a broken collarbone for much of this season.

Former Buffalo quarterback Matt Cassel hasn't gotten the job done for an offense that's one of the least-prolific in the league, ranking 30th with 17.6 points per game and 27th with a 324.4-yard average. Dallas, which will miss the playoffs for the fifth time in coach Jason Garrett's six years, is 29th with 205.2 passing yards per contest.

Cassel completed 58.3 percent of his passes for 1,276 yards with five touchdowns and seven interceptions in eight games. He was pulled after going 3 for 8 for 37 yards and an interception in a 19-16 loss to the New York Jets on Dec. 19.

Moore was picked off three times but managed to help put Dallas in front with a 10-yard scoring pass to Dez Bryant in the second quarter.

"I think you just kind of stay ready and that's the way you've got to approach it," he said. "If that opportunity ever came, you've got to be ready. And fortunate for me, we have that here."

The Cowboys (4-10) have already clinched their worst finish since going 6-10 in 2010. They haven't lost 11 games since doing so in three consecutive seasons 2000-02.

Dallas' loss last weekend was its sixth this season by seven points or fewer with two of those coming in overtime.

"We all start with aspirations," Garrett said. "For some teams it works out and you get to play in the playoffs. For us this year that is not going to happen. But that doesn't diminish the work everyone has put in."

Jason Witten can set a franchise record by catching a pass in a 118th consecutive game, breaking a tie with Hall of Fame receiver Michael Irvin, though he was limited in practice this week due to a chest injury. He's set to play but Bryant won't because of a foot injury.

Buffalo's LeSean McCoy won't be available due to an injured right knee, and one of its best receivers, tight end Charles Clay (back), will also sit out after missing a 35-25 loss in Washington last Sunday.

Another top pass catcher, Robert Woods, was placed on season-ending injured reserve Tuesday (groin) along with linebacker Randell Johnson (ankle). Offensive tackle Seantrel Henderson was put on the reserve-non-football illness list.

Buffalo quarterback Tyrod Taylor is just 27 rushing yards shy of breaking Doug Flutie's team mark of 476 set in 1999.

Dallas has won its last three games against the Bills and routed them 44-7 in the most recent matchup Nov. 13, 2011.