Manchester United manager David Moyes has warned the officials about divers ahead of the visit of Bayern Munich in the Champions League.
The European champions are hosted by United at Old Trafford on Tuesday with Arjen Robben expected to start for the German side.
The Dutch winger was heavily criticised by Arsenal manager Arsene Wenger in the previous round for perceived play-acting, and Moyes, while refusing to name players individually, has said that the referees must be wary of simulation.
He told reporters: "We trust the referees to make the right decisions. Diving is one of the hardest things for referees to get right.
"I would have retrospective cards but I wouldn't want to comment on individual players."
The second leg of the quarter-final tie will be played on April 9.
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