Head coach Erik ten Hag says that some of his Manchester United players "stopped following the rules" as they went down to a 3-2 defeat to Borussia Dortmund on Sunday night.
Last weekend, United recorded a win over Premier League rivals Arsenal as part of their tour of the United States, but the latest setback represents their third defeat in four games across the pond.
The Red Devils had initially taken the lead through Diogo Dalot before two goals in the space of as many minutes from Donyell Malen gave the Bundesliga giants a half-time advantage.
Antony got United back on level terms shortly after the restart, but a Youssoufa Moukoko strike 19 minutes from time earned Dortmund a boost ahead of their own campaign.
When speaking about the match to MUTV, Ten Hag was full of praise for the manner of the performance that occurred during the opening hour of the contest at the Allegiant Stadium.
Nevertheless, the Dutchman had a contrasting opinion about how the players who played the final 30 minutes, including the likes of Casemiro, Bruno Fernandes and Mason Mount, competed in the closing stages of the contest.
Ten Hag said: "Two parts - first hour [was] good, well performing, playing as a team, lot of dynamic, good pressing, so it was really enjoyable to see.
"That is what I wanted to say to the team at half time and then all of a sudden they gave two goals away and they really were giveaways. So that was already annoying but the last half an hour was bad, was a poor performance.
"Didn't follow the rules at all, not in pressing, not in building up and not in attacking, so there were 11 individuals finally on the pitch and that wasn't good at all.
Later in the interview, Ten Hag added: "The first one was really poor defending and the second one really poor in building up and that is not us.
"We have to keep the control and you have to keep the responsibility on the ball and it was the same on the 3-2 for Dortmund, so [it] can't happen and it's unforgivable even if it's a friendly."
United have two more friendlies remaining on their pre-season schedule, a meeting with Lens taking place on August 5 before Athletic Bilbao are their opponents a day later.
Their Premier League campaign begins with a home encounter against Wolverhampton Wanderers on August 14.
