Arsenal were denied a dream start in the FA Cup third round clash with Manchester United when Gabriel Martinelli's opener was disallowed.

The Gunners started fast out of the tracks against their Premier League rivals and were the better side in the opening stages of the contest. After 20 minutes, Mikel Arteta's men had come close on a number of occasions with Kai Havertz testing Altay Bayindir.

Martinelli thought he had opened the scoring in the 19th minute when he calmly slotted the ball past Bayindir. However, after running away in celebration, the Brazilian's joy was replaced by frustration when the assistant referee raised his flag.

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Following good play between Martin Odegaard and Gabriel Jesus, the latter's ball in-between United's defence made its way to Martinelli. The winger raced away and put the home side ahead, but it was disallowed after a small delay.

Arteta was visibly frustrated on the touchline, questioning the decision to the fourth official. Replays showed that Martinelli was in an obvious offside position but Arsenal's frustration came about because Jesus' pass struck Harry Maguire before finding Martinelli.

Despite the ball hitting the United defender on its way through to Martinelli, rules state that Jesus' intention to play the Brazilian through means Maguire's touch doesn't change the assistant referee's offside decision and doesn’t ignore the fact that Martinelli was in an offside position.

The Gunners didn’t let the setback stop them from controlling things at the Emirates Stadium. United continued to struggle to get out of their own half as the first-half went on while Arsenal applied more pressure.