Ruben Amorim defended Altay Bayindir after goalkeeping blunders returned to haunt Old Trafford when the Turkish player’s error allowed Riccardo Calafiori to head in the ­winner for Arsenal.

Amorim was also forced to deny that he had dropped André Onana from Manchester United’s matchday squad, saying the keeper had not yet trained enough after injury.

Bayindir’s mistake came from Declan Rice’s 13th-minute corner, the Turk managing only to palm the ball on for Calafiori to head in. United complained that Bayindir had been impeded – William Saliba appeared to block him – to which Amorim pointed after the defeat.

“If you watch the ball, [the opponent] is allowed to do a lot of things at the corners,” Amorim said. “We need to do the same. But when you touch that way [block], the goalkeeper needs to [be able to] use his hands to catch the ball, not push ­players [out of the way]. He chooses to push players and let the ball pass. But again, it’s the rules. If it’s allowed, we need to do the same thing.”

Amorim was defiant when asked if he considered starting Tom ­Heaton ahead of Bayindir after the latter’s similar error allowed a Son Heung‑min corner to go straight in during the 4-3 Carabao Cup defeat at Tottenham last season. Amorim referred to Bayindir’s next ­display, in the FA Cup third-round 5-3 ­penalty shootout win against Arsenal in January.

“Why? Without VAR [in the compe­tition],” the 40-year-old head coach said. “With VAR, it’s a foul. And then what happened in the next game? Arsenal. Who saved the ­penalty? Who saved the game?”

Manchester United goalkeeper Altay Bayindir flaps at an inswinging corner, palming the ball on for Riccardio Calafiori to score Arsenal’s winner. Photograph: Michael Regan/Getty Images

The Portuguese denied ­dropping Onana, citing his recovery from a ­hamstring problem. Amorim said: “No, I didn’t drop Onana from the squad. I didn’t drop one of the other two from the squad [­Bayindir, ­Heaton]. So it’s not the case of ­dropping Onana. It’s not putting André Onana in just one week without ­seeing the game, without time, with training.”

Amorim was asked if he is content with Onana, Bayindir and Heaton. “We proved today that we can win any game in the Premier League, ­especially against a great team like Arsenal, and we were the ­better team. I’m happy with the three goalkeepers.”

After United signed the No 9 ­Benjamin Sesko for £73.7m, Amorim excluded Rasmus Højlund, another centre-forward, from the matchday squad. He was asked if this meant the Dane would depart. “We’ll see what is going to happen in the market.”

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Of the defeat, Amorim said: “We deserved more and we were the better team, that is clear but in the end we lost. We need to work on the things we need to improve. We need to defend set pieces.

“We did a lot of things right. There are a lot of things without seeing the video that we need to work on. I am really proud of my players. We stuck to the plan even when the stadium was ­nervous, we continued to play the way we played and that will help us in the future.”

Mikel Arteta was delighted with the victory. “Big, big result,” the ­Arsenal manager said. “When you feel that they [United] are building something special [and] want to [start] the season in a good way and you are able to come away with a win, I think I’m very happy for our team.”

Arteta was asked about the debut of his new No 9, Viktor Gyökeres. “He did a lot of things very well. Coming away from Manchester United, you win your first game with Arsenal, it’s a good start,” the Spaniard said.