Ruben Amorim's goalkeeping gamble backfired as Riccardo Calafiori's first-half header gave Arsenal victory over Manchester United at Old Trafford.

In their £200m summer rebuild, United opted against bringing in a new keeper.

With first choice Andre Onana not playing a game in a pre-season because of a hamstring injury he suffered at the start of last month, Amorim selected Turkey international Altay Bayindir ahead of veteran Tom Heaton for his side's opening Premier League game.

But Bayindir allowed himself to get trapped on his line as Arsenal midfielder Declan Rice curled a dangerous 13th-minute corner underneath the crossbar. Bayindir could only get a weak hand to the ball and ended up pushing it on to Calafiori, who turned home what turned out to be the winner from less than a yard.

The result was harsh on the home side, who were much improved on last season and carried by far the greater threat in the game, accumulating more possession and having significantly more shots and efforts on target.

However, Arsenal goalkeeper David Raya made a series of excellent saves to keep the home side out, turning away three shots from one United debutant Matheus Cunha and scrambling to his left in the second half to keep out a goalbound effort from another, Bryan Mbeumo.

Rice went close for the visitors with a late free-kick but Arsenal were still able to complete their fifth victory in six league games over United.

The enthusiastic applause from the home fans that followed initial disappointment, at the end of six minutes of injury time that did not bring the equaliser they craved, told its own story about Manchester United's performance.

They recorded 20 shots in a Premier League game against the Gunners for the first time since August 2011.

On that occasion, they scored eight times. This time, there was nothing.

Amorim spent most of last season grumbling about not taking opportunities. Yet this didn't seem like one of those days. Two-thirds of his new £200m strikeforce started the game - and Cunha, who is clearly going to be a crowd favourite because of the fight he shows in every moment, and Mbeumo were excellent.

Casemiro's passing range was superb. Patrick Dorgu was outstanding at left wing-back and Matthijs de Ligt a rock in central defence.

But they conceded far too easily.

It is easy to imagine United goalkeeping great Peter Schmeichel just barging William Saliba out of the way once Rice's pinpoint corner was heading under the crossbar. Edwin van der Sar was far too cute to be caught out so easily.

Bayindir ended up in a position from which he could have no influence. And Amorim, and Onana, who was at Old Trafford for the game, would only watch in despair.

In the brutal world of professional sport, results mask everything. Amorim has now lost 15 times in 28 Premier League games as United manager - that is the fastest a manager has reached 15 defeats in the competition, promoted clubs aside, since Paul Hart with Portsmouth in 2009.

The excitement before the match for Arsenal supporters was that they were going to see their new £64m signing Viktor Gyokeres in action in a Premier League match.

The Gunners have come into this season with a new direct style that they are hoping will get the best out of the Sweden striker, and be the difference in them lifting a trophy during this campaign.

Gyokeres' main attributes are using his power and strength to burst through the centre-backs and boss Mikel Arteta is trying to increase the amount of chances he gets.

However, Gyokeres is lacking in match sharpness and has only been back in full training since signing for Arsenal - after he did not attend Sporting's pre-season while he looked to force through a move.

There were occasions when the 27-year-old was played into promising positions, but it looks as though it will take time for the players to get used to their new system and for him to build up the fitness he needs to become a key part of the Arsenal attack.

It was not the debut Gyokeres would have dreamed about, and the forward was jeered off by the United supporters when he was replaced by Kai Havertz in the 60th minute.

The usually reliable Arsenal defence did not settle and had some shaky moments during the match - Manchester United able to create chances with direct passes behind the visitors' defence.

Calafiori is important to the Arsenal attack due to his unpredictability on the ball but his runs into midfield and further up the pitch left Mbeumo in space and that also caused problems for the Arsenal backline.

Arteta will be pleased to get off to a winning start but there is a lot for the manager to work on before their next game.