With Poland international Tomasz Kuszczak breathing down his neck, Van der Sar told manutd.com: "You always have to be at your best at United. If you are not, someone else is always ready to take your place.
"But when you make mistakes, you look at how to improve and then get on with it."
With United's major rivals all strengthening, van der Sar is more convinced than ever that the Red Devils need to make a fast start in order to safeguard their title.
Nevertheless, nine points in the first week would send out a sign of strength by the men from Old Trafford.
"We must make sure we get off to a good start, like we did last season," he said.
"We have three games in the first week and it is important we do well in them otherwise we can end up under pressure straight away.
"From then on, the team will grow and the new players will learn what is expected of them."