Joseph Randle has been given the majority of plays at the position this season, but he is currently battling an oblique strain and is racing to prove his fitness to the Cowboys coaching staff.
However, Garrett claims that McFadden will be given the starting role even if his teammate is available.
"We use our eyes to evaluate them," Garrett told reporters when asked about McFadden's promotion. "We don't use our ears. We use our eyes. We've given these guys chances. Are they taking advantage of it; are they warranting more opportunity? That's what we tried to do.
"When someone really grabs hold and says, 'Hey, I want to be the guy,' we're going to give him more chances. We'll do that at the running back position, but really we do that at every position on our team."
McFadden ran for 152 yards and scored one touchdown in the loss to the New York Giants at the weekend.