Ten Hag is the frontrunner to secure the head coach role at Old Trafford as the Premier League giants look for a long-term option this summer.
Ralf Rangnick currently sits in the hotseat, but the German is set to move into a consultancy role at the end of the season.
It means the feelers are out for a permanent option with Mauricio Pochettino and Luis Enrique also linked.
Van Gaal is someone who knows the Red Devils well having led the club between 2014 and 2016, and speaking to his fellow countryman, the Netherlands coach has urged ten Hag to consider his next move carefully.
Quoted by Voetbal International, Van Gaal said: "Erik ten Hag is a great coach, and that is always good for Manchester United.
"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."
While Ten Hag continues to be a frontrunner for the Man United job, Darren Bent believes that the club should consider a move for Roberto Mancini, telling talkSPORT, "He is a winner.
"He is someone who has won the Premier League so he has the experience, he's won the Euros, even though I know it's not ended well with the North Macedonia result. But if United throw a ransom, why not?
"Manchester United are the biggest club in the world. I get it, it would be a big deal in winning Manchester City their first Premier League title to then go to United, but we've seen managers [move to rivals] before.
"In these types of circumstances, there's no jobs for him, Manchester United are there and they just go 'You know what, what do you think?'. I think he is better than [Erik] ten Hag, and certainly Mauricio Pochettino.
"He has proved he can do it in this country, I think that is vital. He has got the Premier League experience - I know he ended up leaving, but then he won a massive trophy with Italy in the Euros, so you cannot question his pedigree in winning things."