Aalborg held Celtic to a goalless draw in Scotland while United got the same result at home to Villarreal in the opening round of the Champions League.
Fletcher said: “It is not going to be quite that easy to beat them.
“Aalborg might not be a household name but this is the Champions League.
“It is a top-quality competition and they are a quality side.
“People might have discarded Aalborg but that will only help them because there is no pressure. It is a game we will have to be on top form for.
“It is important we get a win tomorrow. The longer you go without one the more worrying it gets.
“We need to kick-start our Champions League campaign.”