De Ligt, 19, has been linked with numerous European heavyweights, including United, Liverpool and Barcelona, although Paris Saint-Germain are reportedly favourites to sign the centre-back.
Netherlands great Gullit feels his fellow Dutchman should not head to Old Trafford after United missed out on Champions League qualification.
"I don't think Man United, they don't play [in the] Champions League," he said, speaking at the Costa Smeralda Invitational.
"I think he wants to play Champions League. So, that's unfortunate for Man United.
"It's going to be difficult, a lot of them maybe have a UEFA ban, so there's a lot that's going on still. So I hope he makes the right decision for himself."
De Ligt captained Ajax to a domestic double last season, while they also reached the Champions League semi-finals.