Manchester United full-back Brandon Williams could be heard swearing at teammate Tom Heaton after he was blamed for conceding a goal before half-time, according to reports.
Erik ten Hag's side started the better team at the Allegiant Stadium in Las Vegas, and with 24 minutes played, they got their just rewards when defender Diogo Dalot opened the scoring with a wonderful effort from range.
Dortmund, however, capitalised on some slack defending in the final stages of Sunday's first-half.
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In fact, the opportunistic Donyell Malen bagged a quickfire double in the 43rd and 44th minute, with the second goal sparking a 'heated' argument between teammates Williams and Heaton.
That's according to The Mirror, who suggest Williams appeared to say, "It's your f*** up" when goalkeeper Heaton blamed the 22-year-old defender for playing Malen onside,
As you can see from the footage above, a misplaced pass from Victor Lindelof left the Reds in serious trouble.
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And just 18 seconds after equalising for Dortmund, a well-placed Malen received a pass from former United loanee Marcel Sabitzer before tapping in with his second touch.
An animated Williams can be seen venting his frustration moments later.
Manchester Evening News journalist Samuel Luckhurst, who attended the game in Las Vegas, said of the incident: "Got heated all of a sudden. Heaton and Williams had a blazing row after Dortmund's second. Looked like Williams said 'It was your f**k up.'"
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Ahead of Sunday's pre-season clash against Dortmund, Williams said he was relishing the chance to prove himself ahead of the new campaign.
"It's my first pre-season in three years," Williams told Manchester United's official website after their 2-0 defeat to Real Madrid.
"The other pre-seasons I've been injured so I've come back hungry and I'm enjoying the tour, and enjoying getting to know the new players coming in as well.
"It's good to be together as it's about making that team spirit going into the season stronger. It helps when you're on the pitch if you know a player off the pitch as well.
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"So it's good to speak to some of the foreign players, getting to know their culture, and even the younger boys who've come up and done really well."
He added: "Those who've come up might be shy so it's important to speak to them and get that relationship. It's been an enjoyable week so far and we've got one game to go.
"You've just got to get ready for the first game of the season. You never know, you might be starting – football can change so quickly so you have to prepare yourself as best you can and get involved with the team."
Topics: Manchester United, Brandon Williams, Premier League, Borussia Dortmund, Tom Heaton