"It is not going to be quite that easy to beat them," he said. "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."
United have certainly turned their season around in the past fortnight.
Having gone into the Villarreal clash on the back of a sound beating at Anfield, the Old Trafford side had the better of things with the Spaniards without being able to get the all-important goal.
It was a similar story at Chelsea, only this time United were unable to hold a lead that looked like giving them victory.
Confidence was mounting though and subsequent triumphs over Middlesbrough and Bolton mean they will face Bruce Rioch's side in confident mood.
"We are in better form now," said Fletcher, who knows the last time failed to overcome Villarreal on home soil, the Red Devils were embarrassingly knocked out in the group stage.
"When you look at it, Aalborg's result at Celtic was a good one for us.
"But 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."