Slot has just left Feyenoord to take charge of Liverpool.
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Van Gaal told Sky Sports: "It will be a huge job, because (Jurgen) Klopp has done a fantastic job.
"Then it is always difficult as a successor.
"He is a good trainer. I even think he is the best Dutch coach at the moment, together with Peter Bosz."
Van Gaal saw how his former club Bayern Munich had great difficulty finding a new manager and understands that Liverpool is going for someone who has not previously worked at the highest level.
"Slot was feasible for Liverpool, you have to look at that. I think they made a very good choice."