The Denmark international has only been beaten once, in the 1-1 draw against Benfica, in all of his six matches this term.
When asked by MUTV if having two quality keepers is a nice problem to have, Sir Alex replied: "It's not a problem because I think Anders has that little bit more experience. I felt his was the right time to change and we're going to do that with David.
"He's only 20 years of age and is going to be a fantastic goalkeeper, there's absolutely no doubt about that. Having Anders as a back-up in terms of giving the lad a break at the right time, is proving its worth. Anders is an outstanding goalkeeper and has a little bit more experience."