Man United goalkeeper, De Gea, admits they made a big mistake.
The 28-year-old says United's defenders were put off by the linesman's flag before Pierre-Emerick Aubameyang scored.
During the Arsenal vs Man United match, Arsenal striker chipped in after Bukayo Saka pounced on Axel Tuanzebe's misplaced pass. The assistant referee called offside just as the Gabon striker went to dink it over De Gea.
Later VAR check proved that Harry Maguire was, in fact, playing Aubameyang on and the goalkeeper tried to claim he was distracted.
Asked about the situation, De Gea claimed: "Of course. It's easier if the linesman keeps the flag down but also we know VAR is behind it [the decision] and if it's offside, it's offside."
De Gea enjoyed a strong game and was rarely threatened despite the pouring rain at Old Trafford.
He added: "It was a big mistake for us, we gave away an important goal and I think the game was well balanced. They had good chances and we did but the game was at Old Trafford so we have to win it.
"We keep learning, we have a young team and it's time to keep looking forward. This is Manchester United, we play at Old Trafford and we have to win these games. We have a young team but the lads have to react and play better than they have been doing.
"We are a team and have to improve and we are working really hard to do it and we're looking forward to the next game."