The surgery will end Dunbar's season after four games and leave the Cowboys with a large hole to fill in their backfield as well as on special teams.
Dunbar suffered the injury on the opening kickoff of the second half in Sunday's loss to the New Orleans Saints. Dunbar's foot appeared to stick in the turf, and he fell awkwardly backward and was eventually carted off the field.
Dunbar also suffered a partial tear to his medial collateral ligament, according to a source. Cornerback Orlando Scandrick suffered the same injury in training camp and had surgery within a few days of getting hurt.
"He's gotten more opportunities this year," Garrett said. "I think he just shows how versatile and productive a player he can be. You saw that last night, he ran the ball from the backfield, a big run early on in the ball game, he's shown he can catch the ball out of the backfield, he's shown he can line up as a receive and catch the ball and make plays. He's a good return guy. He's just one of those guys when the ball is in his hands he does a lot of positive things for your football team. He's a hell of a kid, and he's got a great future ahead of him. He's going to come back. He's going to handle it the right way. He'll be better than ever."
Dunbar's 21 catches for 215 yards is a career high. He had a 45-yard run on the second play from scrimmage against the Saints and had five carries for 67 yards for the season. He averaged 24.3 yards per kick return and also had one punt return. Before last night's game he was third on the team in special teams tackles, according to the coaches' breakdown.
While Garrett said the Cowboys are always looking to upgrade the roster, Dallas will look to Joseph Randle, Darren McFadden and Christine Michael to make up for Dunbar's absence. Rookie wide receiver Lucky Whitehead could also play a bigger role.
Garrett said Randle and McFadden are equally suited to fill Dunbar's role.
"One of the things we really pride ourselves on as coaches, and I think what [offensive coordinator] Scott Linehan does such a great job of is using the personnel that we have," Garrett said. "Lance was a weapon for us and we tried to use him as a weapon in a lot of different ways to impact our offense and impact the game. To say we have someone in house who is going to do those same kinds of things, I don't know that we have somebody like that, but we have guys in house who can do different things, and hopefully we'll play to those strengths."
Garrett said linebacker Sean Lee, who suffered a concussion in the first half, was doing better Monday but needs to go through the mandated protocol in order to play against the Patriots.