Although Ajax received an initial €40 million before potential further add-ons for the Colombia centre-back, Van der Sar insists the 21-year-old's immediate future should have been in Amsterdam.
Sanchez faces a battle for game time at Spurs, who had the best defensive record in the Premier League last season thanks to their strong centre-back pairing of Belgium internationals Jan Vertonghen and Toby Alderweireld.
Van der Sar thinks it was too soon for the defender, who made his Spurs debut as a stoppage-time substitute in a 1-1 draw with Burnley at Wembley, to consider a move away from the Eredivisie, considering he had spent just a single season with Ajax after leaving Atletico Nacional in his homeland.
"I think with Sanchez, one year was too early, he should have stayed two or three years," the Dutchman told a Soccerex event on Wednesday.
"But sometimes, with the financial power that is looming over us in the bigger leagues, it’s quite difficult for even a big club like Ajax coming from a small league to resist certain transfers."
"I’d rather not [have to] replace them because I want to have as good a team as possible," Van der Sar said.
"But we know at a certain point that this league is not strong enough, so after a few years, hopefully if you play three, four or five years for the club and give us two or three championships and success in Europe then, [in an] ideal world, it is the right step to go forward."