Huge blow for the Dallas Cowboys … star defensive tackle Gerald McCoy just suffered a gnarly quad injury in practice — and the team now says he’s DONE for the 2021 season.

Reporters on scene in Dallas say the defensive lineman — who just signed a 3-year, $18 MILLION contract with the team this offseason — went down awkwardly during Monday’s workout.

According to multiple witnesses, McCoy was stepped on by a teammate in a routine drill — and could NOT put weight on the leg as trainers helped the 6-time Pro Bowler to the locker room.

Cowboys exec Stephen Jones confirmed a short time later that McCoy ruptured his right quad tendon — and will now be headed for season-ending surgery.

32-year-old McCoy was expected to be a key player in the middle of the Cowboys’ defense this season … head coach Mike McCarthy had just praised him earlier this month, saying, “we look for him to be a force inside.”

The Cowboys will now turn to players like Antwaun Woods, Dontari Poe and Trysten Hill to try to fill the void.