Moyes has described that stop as "incredible", while declaring that the Spaniard is now repaying the faith shown in him by previous boss Sir Alex Ferguson despite his tender age.
He told The Mirror: "It is one of the best I have ever seen. It was incredible. I said it on the bench right away, what a save it was.
"That was a turning point. I don't think we deserved to be losing but had we gone 2-0 down, then it would have been a mountain to climb.
"I think he has definitely established himself now. It was brave for Sir Alex to bring him in and there probably were a few seasons when it might have been a bit rocky. Hopefully David will feel the benefit of that in the years to come."
The 21-year-old has now played more than 90 times for United since his switch from Atletico Madrid in 2011.