The Bayern Munich winger is wary of the Red Devils, despite their poor season, and does not think Robin van Persie's absence makes them any less dangerous.
The Red Devils have struggled without Sir Alex Ferguson at the helm, with David Moyes' side sitting seventh in the Premier League and 10 points clear of a top-four spot.
Winning the Champions League looks like United's only chance of securing elite European soccer next season, but the Dutch winger is not underestimating the damage that the English giants could do to Die Roten.
"In a game like this, form goes out of the window," he told the club's official website. "In the previous round United lost 2-0 at Olympiakos 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 quarterfinals 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."
Robben does not think the absence of Robin van Persie — who scored all three goals in the last-16 second leg for Moyes' men — will hugely affect Tuesday's hosts as he stressed they have other quality attackers.
"I spoke to Robin a little bit since the draw, but not much because of his injury," he added. "It's a pity he will not be playing. That goes for Thiago [Alcantara, who picked up an injury in Bayern's 3-3 draw against Hoffenheim on Saturday] too now. In the final phase of the season everybody wants to be fit and if they're not it's painful, I know from experience.
"Of course Van Persie has a special quality, but they have other forward options like Wayne Rooney, Danny Welbeck, Nani and Antonio Valencia. As I said, it will be a tough duel but we are eager to face them."