Erik ten Hag cancelled a planned day off for his Manchester United squad following the defeat at Brentford and told the players to make up for being outrun by their opponents at the Gtech Community Stadium during the extra session, sources have told ESPN.
Ten Hag's team were due to be off on Sunday but the rest day was cancelled by the manager in response to Saturday's 4-0 defeat.
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He ordered his players to cover 8.5 miles during training at Carrington, pointing out that Brentford had run 8.5 miles more than they had during the game.
Thomas Frank's team covered 67.9 miles across the 90 minutes while Ten Hag's players registered 59.4 miles.
Sources have told ESPN that Ten Hag was at his desk in his Carrington office early on Sunday morning after being "horrified" by United's performance against Brentford.
He branded the first-half display -- during which United conceded four goals in 35 minutes -- as "unacceptable" and afterward questioned how Brentford had managed to show more "hunger."
Back-to-back defeats to start the season has left United bottom of the Premier League table.
Liverpool are their next opponents at Old Trafford on Aug. 22.
"I think we all have seen the game and I think Brentford was more hungry," Ten Hag said.
"We have to question that and we will do. They are our players and we are together and we are in a really difficult situation, it's clear, and we have to find solutions.
"I think they follow my instructions but they make bad decisions and that's football, it's a game of mistakes and you get punished."
Meanwhile, sources have told ESPN that United have denied reports claiming that the club is likely to terminate Cristiano Ronaldo's contract.
The Portugal international has been heavily linked with a move away from Old Trafford this summer but the club remain adamant that he will not leave.