Ten Hag is one of the frontrunners to take over permanently from Ole Gunnar Solskjaer, who was sacked last year.
Ralph Rangnick has been placed in interim charge until the end of the season.
Ten Hag has transformed Ajax into one of the most entertaining sides in Europe since taking over in December 2017, winning two Eredivisie titles and reaching the Champions League semi-finals in the 2018/2019 season.
"Erik ten Hag is a great coach and that is always good for Manchester United," said Van Gaal, who is currently in charge of the Netherlands national team for a third time.
"But Manchester United are a commercial club, so it's a difficult choice for a coach. He'd better go to a football club.
"I'm not going to advise him, he'll call me himself. But he must choose a football club and not a commercial club.'
Van Gaal knows a thing or two about Man United, where he was in charge for two seasons between 2014 and 2016, before being replaced by Jose Mourinho.
The 70-year-old was sacked two years into a three-year contract. His dismissal came just two days after lifting the FA Cup.
United finished fourth and fifth in Van Gaal’s two seasons in charge.
