Vincent Kompany's contentious sending-off in the Manchester derby on Sunday did not so much spoil the third-round FA Cup tie as define it. From there, in the 13th minute, Manchester United for a period resembled champions, before Manchester City found admirable resilience.

Unsurprisingly, City yesterday appealed against the red card that Chris Foy deemed Kompany's challenge merited. Those in the television studio were largely in agreement with Robert Mancini, who could barely bring himself to discuss the incident in the immediate aftermath of the game, such was his displeasure with it. Only when Sir Alex Ferguson unanimously backed the decision did the technical debate begin. "I think if he catches Nani then he [Nani] has got a problem," said Ferguson. "I've seen him do it before, he maybe got off in the past."

Kompany's tackle did not carry the malice that, say, Karl Henry's did at Wigan last season, when he slid in at full pace with both feet off the ground. His red card was instant, the decision was backed by Mick McCarthy and Henry, then Wolves captain, apologised. For that tackle, there was not even a suggestion of controlled aggression. With Kompany, it did not look like he was attempting to do anything other than win the ball.

However, the legitimacy of such a challenge is still defined by the rule book. Kompany was guilty of serious foul play. It is not enough any more to say simply that he won the ball.

These are the laws set down by Fifa. "Any player who lunges at an opponent in challenging for the ball from the front, from the side or from behind using one or both legs, with excessive force and endangering the safety of an opponent is guilty of serious foul play."

If, as Ferguson asserted, Nani had not subtly lifted his leg, then the tone of the argument could have changed dramatically. Nani could have missed the rest of the title run-in. The safety element of the player has become paramount, and that has been a deliberate policy led by the PFA.

It was at Manchester City last season that Nigel de Jong snapped Hatem Ben Arfa's leg in two places with his trailing leg, also in a sliding tackle. To the fury of Newcastle officials, De Jong was not sent off, nor did he suffer through retrospective evidence because Martin Atkinson, the referee on the day, had seen the incident. It would be curious to see what Foy would have made of such a tackle.

Rio Ferdinand and Wayne Rooney would be expected to back the sending-off of Kompany. "By the way, how can there be any debate about the red card yesterday??" tweeted Ferdinand. "You leave the ground with a two-footed tackle= red card, fact."

But Jason Roberts backing the official perhaps offered a centre forward's perspective on this. These are the kind of tackles that, when they go wrong, cause serious, career-threatening injuries. There are three key elements to an instant red card on the grounds of serious foul play. Kompany's challenge, although not apparently motivated by malice and which did see him win the ball, qualifies on each account. The force was excessive, a player's safety was endangered (Nani's) and, as it was a sliding tackle, thus a lunge.

Ultimately, it was not Foy's decision that defined such a crucial game and its outcome; that was down to a tightening of the rule book (led by the association that protects professional footballers) and Kompany's reckless challenge. As was seen in the dismissal of Nenad Milijas against Arsenal and the subsequent failure by Wolves to have the decision overturned, the rule book is tightening.

The huge irony here is that it will be former players who shout the loudest about the diminishing sight of players sliding into each other, yet it is for the safety of those playing the game now that the law is being strengthened and upheld.

The last word: what the rule book says

Law 12 - Fouls and Misconduct

Serious foul play: A player is guilty of serious foul play if he uses excessive force or brutality against an opponent when challenging for the ball when it is in play.

A tackle that endangers the safety of an opponent must be sanctioned as serious foul play. Any player who lunges at an opponent in challenging for the ball from the front, from the side or from behind using one or both legs, with excessive force and endangering the safety of an opponent is guilty of serious foul play.

Advantage should not be applied in situations involving serious foul play unless there is a clear subsequent opportunity to score a goal. The referee must send off the player guilty of serious foul play when the ball is next out of play.

A player who is guilty of serious foul play should be sent off and play is restarted with a direct free-kick from the position where the offence occurred (see Law 13 – Position of free-kick) or a penalty kick (if the offence occurred inside the offender's penalty area).

Tweet: Reds have # say

Rooney "Funny how people think i got kompany sent off. Im not ref. i didnt give red card. But it was a clear red card. 2 footed tackle."