Coleman will pit his wits against the new United boss this week when his Wales side take on Holland in an international friendly in Amsterdam on Wednesday.
United have turned to Van Gaal, who is stepping down as Holland coach after this summer's World Cup, to revive their fortunes after a disappointing season under David Moyes.
Coleman told Press Association Sport: "It will be interesting to see what happens if Manchester United go through next season not winning anything because there is that possibility.
"Regardless of how easy they look on the eye if they haven't won anything what are they going to do?
"They are used to winning things and bossing situations and last season they didn't do that.
"I thought it would be hard for Moysey in the first year or two and I was disappointed as a fellow manager that he only got 10 months.
"I want to see what happens next season because Louis van Gaal is a quality manager but what are we going to say next year if Manchester United have not won anything?
"Is Louis van Gaal going to be out the door because he is going to need time to bed in with his own ideas.
"You are not a magician when you are a manager
Sometimes you go in and have an immediate response and that is good but sometimes you don't and it takes time - if you get the time - to try to mould it and do it your own way.
"That was what David Moyes was trying to do and I was disappointed because he is a British manager and we want to see them do well like Brendan (Rodgers) at Liverpool.
"He had a season and then he got it right.
"Van Gaal with all his experience I am quite sure he can do well there but it will be interesting to see how it works."
Coleman has the highest respect for Van Gaal, 62, who has also managed - and won titles with - European giants Ajax, Barcelona and Bayern Munich.
Coleman is hoping his side can give the Dutch a good test in what will be a final run out for Van Gaal's men ahead of the World Cup.
Holland are due to fly out to Brazil on Thursday.
Coleman said: "I've never done battle with him, so to speak, but I have been on a couple of seminars when he's been there and he is a good manager.
"He is very innovative
He is one of those managers where it is how it is, that is what he believes and you go along with the ride with him but if you don't agree then you are not on the same journey and I think you find yourself on a different journey pretty quickly.
"I have been doing this for 10 years and it is a hard industry and the ones who are the best managers, the most successful, I look at them and I have a lot of respect and admiration for them.
"He is up there because he has won all the big things and he has been at a lot of big clubs and now he is going to go to one of the biggest clubs and he is someone I respect a lot."
Wales are without some of their main stars, most notably Gareth Bale, Aaron Ramsey, Ashley Williams and James Collins, but Coleman still feels the match is a useful exercise for this squad.
The uncapped trio of defenders James Chester and Paul Dummett, of Hull and Newcastle respectively, and Manchester United midfielder Tom Lawrence could be among those to feature at the Amsterdam Arena.
Wales' next match in September will be their opening Euro 2016 qualifier in Andorra.
Coleman said: "My point is we need to give all of these guys experience of playing international football because you never know when you are going to have to call on them."
Source : PA
Source: PA