Erik ten Hag labelled Manchester United's 3-1 defeat to Aston Villa as "unacceptable" as Unai Emery enjoyed a dream start to life as Villa boss on Sunday.
The Red Devils were unbeaten in nine games before their trip to Birmingham as Ten Hag's project at Old Trafford was beginning to take shape.
But the visitors were blown away in a bright opening from the hosts as Leon Bailey and Lucas Digne struck inside the first 11 minutes.
Luke Shaw's strike deflected off Jacob Ramsey into his own net just before half-time to give United life.
But Ramsey restored Villa's two-goal cushion four minutes into the second half with a cool finish from Ollie Watkins' cut-back.
"You have to read the game and get right the organisation. Not concede two goals -- that's totally unnecessary," said Ten Hag of his side's start.
"It's not acceptable that you concede a goal so quickly in the second half on the counter-attack.
"Our players are experienced. You have to win your battles.
"It looked like they were fresher. That's not acceptable. Players have to take responsibility and be ready."
United missed the presence of the injured Raphael Varane, Jadon Sancho and Antony, as well as the suspended Bruno Fernandes as defeat leaves them three points adrift of the top four in fifth.
The absence of Fernandes and decision to leave Harry Maguire on the bench saw Ten Hag's hand Cristiano Ronaldo the captain's armband just weeks after the Portuguese was disciplined for refusing to come on as a substitute against Tottenham.
Ronaldo has still just scored once in the Premier League this season and endured another frustrating afternoon, which could have been even worse had a VAR review for violent conduct not deemed there was not enough in his clash with Tyrone Mings to justify a red card.
By contrast, Emery could not have wished to have made a better first impression on the Villa support with a first home league win over United since 1995.
"The supporters were amazing supporting us and the players were great with the plan," said the Spaniard.
"The way we played the 90 minutes we can be optimistic but only first step and we have to work a lot to keep improving."
Victory lifts Villa up to 13th and three points clear of the relegation zone.