The 38-year-old is in his fourth season at United, since joining the club from Fulham.
Van der Sar told The Sun: “If I had stayed at Fulham I would have quit the game by now.
“After my third and fourth year there, I thought my chance of playing for a big club again had gone.
“I went to Fulham for the project they explained to me — but it didn’t really work out.
“I might have been playing at a high level for the national team but I was starting to miss those European nights and challenging for titles.
“At certain times you start to — maybe not lose the faith — but just wonder how long it will take until someone picks me up.
“I was thinking, ‘Is it just me? Do I think I’m better than any manager does? Do they think I am just too old or not good enough?’
“But Alex Ferguson bought me and I have to admit, if I had not been at Manchester United, I’d have stopped playing by now.”