Former Premiership ref Graham Poll admits match officials are aware that Manchester United star Cristiano Ronaldo is targeted by the opposition.

In his column for the Daily Mail, Poll wrote: "Sir Alex Ferguson was right to identify Villarreal's tactic of 'systematic' foul tackles on Cristiano Ronaldo but what can a referee do about it if other teams adopt the same approach?

"Italian official Roberto Rosetti did what he could to protect Ronaldo on Tuesday night, cautioning two Villarreal players and dismissing another for fouls on the United forward.

"However, it is not the duty of the referee to single out 'skilful' or 'creative' players for special protection - rather, he should look out for team tactics to foul one opponent continually and clamp down on this."