Bayern host United in Munich with Dutch winger Arjen Robben, their best player this season, in doubt with a minor calf muscle injury, and wary of the threat posed by United striker Wayne Rooney
Yet Van Gaal sounded confident the Bavarians can match their opponents' quality for at least one match.
"We can surprise every team, I believe that. Bayern are not yet a top level team like Manchester but we can achieve that level for a game," the Dutchman said.
Van Gaal said Robben would not play unless he was fully fit, after picking up the injury in their 2-1 defeat to VfB Stuttgart on Saturday.
"I think we have to be careful with a player who makes so many dribbles with such intensity in one game," Van Gaal said.
"It is clear that he is not injured but his muscles are tired and I have to be certain that a player is 100 percent fit. This is a big match and we will try everything."
"We need a player like Robben. But if he is not 100 percent fit he will not play."
SUPER ROONEY
For all Rooney's goals this season, the United forward has scored 33 so far including four in his last two Champions League matches, Van Gaal said if Bayern play as a team the England striker may find the going tough.
"I think that Rooney is difficult to contain but maybe we can do that as a team. We have to do it as a team," van Gaal said, wary that the 24-year-old has come of age.
"Wayne Rooney is a super player and when he was younger I wondered if he had vision in his game.
"He has developed that and that makes a player a great player. He has this vision now."
Van Gaal feels if Bayern match Manchester's on-pitch discipline they are in with a fighting chance.
"I know one thing and that is that the discipline at Manchester is always very good. I am jealous of this discipline and if we have that tomorrow we have taken a big step forward."
Apart from Robben, Van Gaal will also be without suspended midfielder Bastian Schweinsteiger, while striker Mario Gomez has recovered from a muscle injury but will sit on the bench.
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