Bayern Munich winger Arjen Robben believes Manchester United have the ability to punish the reigning European champions if they are not at their best on Tuesday.
The two sides meet at Old Trafford in their Champions League quarter-final first-leg tie.
United have endured a poor season by their standards, with David Moyes's side seventh in the Premier League.
"It's going to be a tough duel and we've got to watch out," Robben said.
"The media and public opinion is that Manchester are not in good shape and that we're going through to the semis, but I don't like that at all."
Bayern head into the game boasting a formidable record of having lost just once in all competitions in the last 12 months.
That defeat was against Manchester City in the Champions League group stage, when Manuel Pellegrini's side won 3-2, although both teams had qualified for the last 16 at that stage.
Robben says he and his team-mates need to be at their best for the whole 90 minutes on Tuesday, or risk returning to Germany with a first-leg defeat.
"You only need to get distracted for a second and you lose against Manchester United," he added in an interview for Bayern's official website.
"If you lose concentration and lose the ball, then you get punished there."
Robben's volley at Old Trafford sent Bayern into the 2009-10 semi-finals at the expense of United.
The German side lost that game 3-2 but Robben's late strike meant the tie finished 4-4 on aggregate, sending Bayern through on away goals.
Credit: The BBC
