Manchester United have announced that Amad Diallo missed Sunday's 3-2 friendly defeat to Borussia Dortmund due to a knee injury.

The Ivorian attacker - who excelled during a loan spell with Sunderland last season - was seen on crutches after sustaining an issue in a 2-0 win over Arsenal during the Red Devils' United States tour.

Diallo appeared to suffer the injury following a tackle by Arsenal defender Jurrien Timber, and he played no part in a subsequent 3-1 defeat to Wrexham, where a youthful Man United side took to the field.

There were fears that the 21-year-old could have been facing a lengthy period on the treatment table, but Diallo recently claimed that the issue was "not serious" and he was back on the bench for the Red Devils' 2-0 loss to Real Madrid.

However, Diallo was nowhere to be seen on Sunday evening, and Man United have now confirmed that the ex-Atalanta BC man is nursing a knee issue which requires further assessment.

At this stage, it is unclear how severe Diallo's problem is, but it is an unwelcome setback for the 2002-born attacker as he bids to finally ignite his Man United career.

Diallo commanded a €21.3m (£18.3m) fee to leave Atalanta in 2021, but he has only played nine times for the Man United first team, scoring one goal and setting up another.

The Ivory Coast international played a crucial role in Sunderland's unsuccessful push for Championship promotion last season, though, netting 14 times as the Black Cats reached the playoff semi-finals.

Diallo's blow comes after another highly-rated youngster in Kobbie Mainoo was ruled out for the first part of the 2023-24 season, having picked up an ankle injury against Real Madrid.

Eighteen-year-old Mainoo had earned plaudits for his performances in pre-season and has been tipped for a breakthrough campaign in the first team this year, but he and Diallo are not alone in the infirmary.

Argentinian attacker Alejandro Garnacho and French defender Raphael Varane also sat out the defeat to Dortmund, although Man United described the pair's absences as precautionary.

Man United drew first blood against Dortmund in Las Vegas through Diogo Dalot, but Donyell Malen's double propelled the Bundesliga side into a 2-1 advantage before the half-time whistle sounded.

Antony's second-half response proved inconsequential as Youssoufa Moukoko scored the game's fifth and final goal, and Erik ten Hag blasted his side's "unforgivable" display after the game.

Man United now return to Old Trafford to take on Ligue 1 high-flyers Lens on August 5, one day before their final exhibition game against Athletic Bilbao in Dublin.