The Germany international is surprised to see David Moyes' side struggling domestically but still feels it will put up a good fight in the Champions League.
The Bavarian giants head into the game having already been crowned Bundesliga champions and remain the favorite to win the Champions League this season, while Tuesday’s opponents are struggling in the Premier League as they currently sit in seventh place.
But the Germany international has warned that his club must not lose focus against David Moyes' men at Old Trafford.
"Everyone knows what to expect tomorrow," he told reporters in a press conference. "It will be an incredibly difficult game. It's something very special to be able to play at Old Trafford."
He added: "It's great to have already one title, but we can't lose focus now. We want to win every remaining game. We're a team that wants to win every game - even in training. This is no different."
The midfielder was then asked for his thoughts on the Red Devils' poor domestic form, to which he replied: "United should be one of the top four in the league all the time."
Bayern goalkeeper Manuel Neuer also feels United will give the visitors a difficult test at Old Trafford.
"The name Manchester United deserves a lot of respect alone. They fought back well against Olympiakos," he said. "We know just how well they can maintain their focus."