Defense has been a major issue for the Kansas City Chiefs in 2021. KC will need its defense to step up against the potent Dallas Cowboys' offense in Week 11 if Andy Reid and Co. want to avoid losing their fifth game of the season.

Unfortunately, things may just have gotten more difficult for the defensive unit, who may have to take the field Sunday without one of its leaders and most important players.

According to independent NFL reporter Dov Kleiman, star safety Tyrann Mathieu was added to the injury report and is considered questionable for Sunday's showdown with the Cowboys. Mathieu is dealing with a knee injury and will be a game-time decision ahead of the afternoon kickoff.

The Chiefs have surrendered 373.2 yards per game this year, the sixth-worst in the NFL. A matchup with the Cowboys, who boast the league's highest-scoring offense (31.6 points per game), was always going to be a struggle. But with Mathieu potentially unavailable, the already frail secondary is looking even more vulnerable.

If Mathieu isn't able to suit up, the team may look to Daniel Sorensen to fill the gap at safety, especially considering Armani Watts is also dealing with an injury of his own. Neither presents much case for optimism from Chiefs fans, who will surely be hoping down to the wire that Mathieu's knee won't keep him out of action.

Mathieu has played in nine of the Chiefs' 10 games this season, having missed the season-opener against the Browns due to an illness. The three-time Pro Bowl safety has two interceptions, five pass defenses, and 46 tackles in 2021.