A court in Haarlem ruled that because Johan Cruyff, one of the club's five commissioners, was not consulted about the decision taken by the other four, it could not go ahead.
Instead, the judge ruled, the decision was on hold for three months until the club's shareholders meet and vote on the issue.
A rift has opened up between Cruyff, now a board member but revered as Ajax's greatest ever player, and the others who include former Dutch international Edgar Davids.
Cruyff has developed a plan to rejuvenate Ajax's famous youth training programme with former players including Dennis Bergkamp, Ronald de Boer, Jaap Stam and Marc Overmars playing a leading role as trainers in the academy.
The court decided that the naming of Van Gaal and also Martin Sturkenboom as directors was legal though the judge suspended the decision for three months to give shareholders the chance to back Cruyff's plans for the club, or those of the other four board members.
