The Spanish stopper has overcome a shaky start at Old Trafford to become a steady influence in between the posts, but the Dane believes that De Gea can become better still if the team's backline is bolstered.
Schmeichel told BBC Sport: "For Manchester United's David de Gea to become a more dominant goalkeeper, I think that he needs a better defence in front of him.
"For a couple of years now Manchester United has lacked a strong, reliable rearguard that plays higher up the pitch.
"That gives De Gea plenty of time to show off his shot-stopping, but it is difficult to command your area without the understanding and confidence in the players in front of you."
De Gea kept a clean sheet on his first competitive appearance for Spain in a 4-0 win over Luxembourg earlier this month.