Football fans have been forced to re-examine a rule they thought they understood after a controversial equaliser put United on the path to a comeback win in the Premier League's Manchester derby. 

Having gone behind 1-0 on the hour mark after Jack Grealish scored for City just after coming off the bench, United initially had an apparent equaliser ruled out for offside.

Forward Marcus Rashford mistimed his run and had run well in behind the City defensive line when Casemiro sent his long ball through from the right wing.

While Rashford danced around the ball without touching it, the linesperson's flag still went up to call him offside as Bruno Fernandes swooped from the left to score 12 minutes from full-time.

But VAR intervened and ruled that Rashford had not touched the ball or interfered with any defenders, despite making an incisive run and coming within centimetres of connecting with his boot.

"The first goal is a joke that it can be allowed," Manchester City defender Manuel Akanji told BBC Sport.

Rashford was undeniably involved four minutes later, when he knocked in the game-winner from close range, for his ninth successive goal in matches at Old Trafford.

City manager Pep Guardiola was not happy with the VAR, implying that the game being played at United's home ground had something to do with the result.

"Rashford is offside, Bruno is not offside, the question is interference," Guardiola said.

"When one player on the edge of the 18-yard box shoots and one player is in front of the keeper but doesn't touch the ball, it is disallowed all the time.

"The decision is they didn't believe Rashford was intervening in this stadium. This is Old Trafford, we have to play much better, like Anfield, we have to do better."

According to English football's governing body, players should be penalised if they come from an offside position and interfere with an opponent by:

• preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent's line of vision or;
• challenging an opponent for the ball or;
• clearly attempting to play a ball which is close when this action impacts on an opponent or;
• making an obvious action which clearly impacts on the ability of an opponent to play the ball

Former England defender Graeme Le Saux said on NBC that the rule was applied correctly, but it needed to change.

"As a defender, anyone on the pitch making a forward run is affecting what I do," he said.

"It's very marginal from the point of view of 'how much impact did he have?' I think they always like to simplify that; if it's not obvious then they're not going apply the law there.

"It's something that's always annoyed me because I think it doesn't take into account the positions we get into as defenders … so of course it impacts in some ways.