It's all down to the new interpretation of the offside law.

Man Utd's top striker was in an offside position, but the flag stayed down because players can now operate as long as they don't interfere with play, an opponent or gain an advantage.

Premiership referee Graham Poll explains the new interpretation.

In November 2003 Fifa reinforced the idea that being in an offside position is not an actual offence.

The law now states that a player in an offside position is only penalised if he/she is involved in active play by:

• Interfering with play
• Interfering with an opponent
• Or gaining an advantage by being in that position

That means that when a ball is first crossed a player can stand in an offside position as long as he isn't doing any of the three things above.

In this case, when the ball was crossed, Van Nistelrooy was in an offside position but ultimately he wasn't interfering with play because the ball didn't reach him.

The ball was headed away so that phase of play is now over.

Sam is an opponent of the new rule

If the ball had skimmed off a defender and Ruud had touched it, then he would rightly have been flagged offside.

The question we now have to ask ourselves in games is: Is he or she really interfering?

Second, he didn't prevent a Southampton defender playing nor did he get in the goalkeeper's line of vision so he wasn't interfering with an opponent.

So was he gaining an advantage by being taking up an offside position?

The answer is no. Had the ball hit the bar or the post and rebounded to him, he would have been penalised.

It has made the job harder for both us referees and the assistant referees but we're getting better at it.

Now the assistants have to judge where the attacker is when the ball is kicked and now wait and see where and with whom the ball ends up.

We have to adopt a policy of, 'wait, wait and see.'

I'm surprised something hasn't been made of this before but it usually takes a high profile game at Old Trafford for something like this to be brought to attention.