The Uruguay international is a target for United boss Sir Alex Ferguson.
But former United midfielder Muhren told the Manchester Evening News: "Unlike (Ruud) Van Nistelrooy there is a question mark for me over Suarez.
"There is no doubt that Suarez is a great goalscorer in the Dutch League. He's proved it. But I am not sure he can score the same amount of goals outside of Holland.
"Scoring goals is very, very important for him. That's his mind set and for an attacker that's a great mentality to have.
"But it depends on what teams want from their forward. Luis isn't a combination player. He isn't a one touch player. When he gets the ball his sole thought is to beat a man and try and score.
"If that is want you want then he is worth looking at. I am sure a lot of clubs like United will be taking a very serious look at him in South Africa.
"If he has a good World Cup and scores goals on that stage then he could convince a few he is worth buying."