The Red Devils have struggled this season as they sit seventh in the Premier League, 10 points adrift of the Champions League qualification places.
However, Robben has warned that Bayern will not underestimate their opponents at Old Trafford.
"In a game like this, form goes out of the window," he told Bayern's official website. "In the previous round United lost 2-0 at Olympiacos then won at home 3-0.
"We must be careful. In the media and the public it's general consensus that Manchester United are not in good shape, but they have reached the quarter-finals here.
"But I do not think like this - this is a difficult duel! We need to perform from the first minute and put in a top performance. You need to be careful or you can lose against United."
The return leg will take place at the Allianz Arena next week.