Mikel Arteta said his Arsenal team “deserved to win by a mile” as they fell to an “unbelievable” FA Cup defeat at home against Manchester United.

Arteta’s side once again wasted a series of presentable goalscoring opportunities, including a second-half penalty, as 10-man United eventually sealed victory through a shoot-out.

The Arsenal manager said the outcome was effectively a one-in-a-thousand result as he told his players they must learn to “batter” their opponents.

“It is unbelievable,” said Arteta. “You measure the performance and what we did, you deserve to win the game by a mile. But the reality is we are out and the only thing that is going to be judged, but internally I can’t [judge it that way].

“I love my players. I love our team and I love how good they are and what they do. Because in 1,000 games you should lose one and probably it was this one. You have to understand this as well. It is part of our industry, our game.

“There is an element that is about putting the ball in the back of the net, that we did once. The amount of situations, chances, penalties that we had we didn’t [convert].”

Sunday’s loss marked the second consecutive game in which Arsenal have been punished for their wastefulness in the opposition penalty area, following their midweek League Cup defeat by Newcastle United. They scored just once in those ties despite having 49 shots.

The two matches have further increased the pressure on the club to sign a new forward during this January window, with that need seemingly even more pressing now after Gabriel Jesus suffered a potentially serious knee injury against United.

Jesus, who has already suffered numerous knee problems since joining Arsenal from Manchester City, was taken off on a stretcher in the first half in what Arteta described as a “big worry”.

“He had to come off on a stretcher with a lot of pain, touching his knee,” Arteta added. “It’s not looking good. The worrying factor is the feeling that he had.”

United’s official X account posted the word “justice” after the full-time whistle, seemingly in reference to the controversial penalty that Arsenal’s Kai Havertz won in the second half.

Havertz went down under a challenge by United defender Harry Maguire, who screamed furiously at the Arsenal forward. The penalty incident also triggered a melee on the pitch, in which the United players Manuel Ugarte and Alejandro Garnacho aggressively confronted Havertz.

United captain Bruno Fernandes and head coach Ruben Amorim praised the team’s ability to “suffer” and “sacrifice” as they demonstrated huge defensive spirit against Arsenal’s onslaught.

The victory was capped by summer signing Joshua Zirkzee scoring the winning penalty just two weeks after being booed off the Old Trafford pitch in a humiliating first-half substitution, with Amorim showing his faith in the striker by selecting him in the decisive fifth spot-taker slot.

“It was a sacrifice for my players,” said Amorim. “The players fought. Sometimes what we talk about before the match is not tactical or technical, it’s this. We have to compete no matter what, no matter what the situation. We competed today. We deserved to win, Arsenal didn’t deserve to lose but we deserved to win.

“Sometimes we have the feeling that everything in the end is going to be OK. I felt that since the beginning of the second half, even after the penalty. I think the Arsenal team felt that maybe today is not their day.

“We were facing a great team and it was not just 90 minutes, it was more than that and then you feel like the players are really tired. They put in new players, they controlled the ball all the time, it was a sacrifice for my players. But we deserved to pass through this round because we suffered all together and we showed character and that’s a very important thing.”

United’s task was made even more challenging by the red card for wing-back Diogo Dalot, who was dismissed for a second bookable offence shortly after half-time.

“Obviously the circumstances of the game — we played with one less,” said Fernandes, who scored a beautiful opening goal for his team. “We suffered a lot but we sacrificed a lot as a team and, at the end, you get this [victory].”

The reward for United’s victory was a fourth-round tie at home to Leicester City, managed by former United striker and coach Ruud van Nistelrooy. Arsenal, meanwhile, are under huge pressure to turn their fortunes around when they meet rivals Tottenham Hotspur on Wednesday.