Pep Guardiola believes Manchester United's first goal against his City side should have been ruled out for an offside by Marcus Rashford, as the forward was "intervening" in the move that saw the derby turn on its head.

City saw their 1-0 lead at Old Trafford overturned when Bruno Fernandes curled home before Rashford netted the winning goal three minutes later - but the first goal was clouded in controversy.

Casemiro played a ball forward towards Rashford - who was standing in an offside position and tried to get onto the pass. The United forward then realised Fernandes - in an onside position - was better placed and the midfielder ran on to score.

Rashford did not touch any player in the move, but did make a clear attempt to get onto the end of the pass.

Guardiola believes Rashford was affecting play by interfering with City defender Manuel Akanji's defensive run, which would make him offside according to the rulebook.

"Manu Akanji stops the line [of running]," said Guardiola. "If he knows that he is going to go with Rashford, he will go back with him and do the duel one versus one and he sees what happens. But he [Rashford] intervenes in the action.

"But the referee decided he did not intervene in this stadium. It's ok, what are we going to do? Are we going to make a complaint? No. Congratulations for that. Sometimes it happens in our [favour]. Sometimes [it's like] that.

"The decision is they didn't believe he [impacted the play]. Rashford was intervening in this action when this happened. It's Old Trafford. We have to play much better. Like it's Anfield. We have to do better.

"The rule is the rule, the interpretation belongs to the referee. I say Rashford is offside, Bruno is not. The situation is for the referees."

Guardiola gave the example that an offside player standing in the line of the goalkeeper's vision from a team-mate's shot is normally penalised - and that this was a similar scenario.

The City manager was, however, more focused on how his side could have defended better in the minutes after Fernandes' equaliser. Rashford tapped home from Alejandro Garnacho's cross just three minutes and 46 seconds after United's first goal.

"Rashford is offside. Bruno Fernandes is not offside. The question is intervening or not intervening in the action," said Guardiola. "When one player shoots and [another] player is in front of the keeper and does not touch the ball, it's disallowed all the time.

"The decision is of course [belonging to] the referees and VAR. We follow the action, we don't make an offside [appeal], we follow the action and after the action, it's either Ederson intervenes [with a save] or not.

"What can we improve? After we concede the goal, in the next few minutes we cannot concede the next one. We allowed them to score. Come, come, come. This is the most important detail we have to improve for the future."