Manchester United manager Alex Ferguson fears Cristiano Ronaldo could suffer a serious injury if referees don't punish players for fouling him.

Ronaldo is serving a three-game suspension following his sending off for head-butting Portsmouth midfielder Richard Hughes in midweek.

Though Ferguson had no qualms with Steve Bennett's decision, he is adamant the United winger was provoked into the act due to the referee's failure to punish earlier incidents.

And the United boss is convinced that last season's Player of the Year could pick up a severe injury, if referees do not clamp down on over-the-top tackles.

"I think that he was falling in the trap of 100 years' experience of football that the perpetrator gets less punishment than the retaliator," said Ferguson.

"Of course, Cristiano has fallen into that trap again, particularly with that referee as two years ago he got kicked all over the pitch against Man City and was sent off.

"It's a bitter lesson for the lad. I don't know what the directions seems to be, but maybe referees are a bit more tolerant of physical contact and I have noticed this in the last few months, with late challenges that go unpunished.

"Whether they get instructions to allow that, I don't know, but I thought these things are black and white. Late challenges should not be allowed.

"We saw it again on Wednesday night, he was letting it go and it went unpunished time and time again.

"Players like Ronaldo are going to be victims, the fans are going to be victims because it will end up with somebody getting a serious injury.

"It's very difficult to completely punish Ronaldo, or be angry with him in a way, because some of the things that happen to him are not right."

When asked if he thought Ronaldo could suffer a serious injury, he replied: "I think there's every chance of that."