Van der Sar told MUTV: "We knew it was a hard place to come. After we took the lead [through Cristiano Ronaldo] they changed their formation to 4-4-2 and created more problems for us. We didn't have as much possession.
"We conceded a sloppy goal and then went behind, but we showed great resilience to come back into the game."
The veteran Dutchman insists it was imperative United avoided defeat and maintained a three-point lead over Chelsea at the top of the table.
"A draw is not bad but losing was out of the question," he said. "We want at least a three-point difference between us and them before we go to Stamford Bridge.
"The boss has already said it's going to be very tight - it's a three-horse race. Arsenal visit on Sunday, but Chelsea have a difficult run-in and it's not certain that they'll win all their games between now and the end of the season."