The Dallas Cowboys received a major blow to their defensive unit as tackle-leader Sean Lee is schedule to have season-ending toe surgery.

The third-year linebacker tore ligaments in his right big toe in the second half of Dallas’ 19-14 win over the Carolina Panthers on Sunday. After trying to play through the injury, Lee would later take himself out of the game.

Dallas has depth at the linebacker position, but replacing Lee will be difficult. Not only did Lee lead the team in tackles, but he was also the defensive signal caller. That role now falls to second-year linebacker Bruce Carter.

Just behind Lee on the depth chart is Dan Connor, a five-year veteran who signed this offseason after four seasons with the Carolina Panthers. Connor is expected to take over the starting position.

Connor filled in for Lee against Carolina and recorded two tackles. He has appeared in five games this season and has 11 total tackles. Connor has also dealt with injuries after two stints on the injured reserve in his first four seasons.

The Cowboys viewed Lee, along with superstar DeMarcus Ware and six-year veteran Anthony Spencer, as anchors to their linebacker corps for the next several seasons.

According to ESPN, Dallas has reached out to veteran Ernie Sims to possibly sign him to fill the void left by Lee. The Cowboys drafted two linebackers in April: Kyle Wilber and Caleb McSurdy.

Losing Lee comes at what could be a crossroads for the 3-3 Cowboys, as they host the NFC East-leading New York Giants on Sunday at Cowboys Stadium.

New York ranks second in the NFL in offense, averaging 411.6 total yards per game and the fourth most points at 29.3 behind quarterback Eli Manning. The Giants have also allowed six sacks this season, the lowest in the league.

The Cowboys, who have not won a game this season by more than seven points, opened the season with a victory over the Giants at MetLife Stadium, 24-17. Lee had a team-high 12 tackles.

Another win over the Giants would provide much-needed momentum for Dallas heading into tough matchups with the Atlanta Falcons and Philadelphia Eagles.