Ruben Amorim admitted Manchester United "need to change" after a dismal 4-3 defeat at Brentford on Sunday underlined the problems facing the Old Trafford boss.
Kevin Schade scored twice at Gtech Community Stadium as United's miserable Premier League campaign took another blow, with their winless league run stretching to six matches.
Amorim made eight changes to his starting line-up as he rested key players after Thursday's 3-0 win at Athletic Bilbao in the Europa League semifinal second leg.
Fielding the third youngest starting line-up in Premier League history – with an average age of 22 years and 270 days – United's raw rookies were unable to avoid a 12th defeat in Amorim's 24 league games.
Since replacing the sacked Erik ten Hag in November, Amorim has been unable to lift United out of the mire.
United are languishing in 15th place as they try to avoid their worst finish since being relegated in 1973-74.
Amorim has been critical of United's players on several occasions and he again conceded there is a need for a close-season overhaul of his squad.
"It was a reflection of our season. We started well and controlled well, we suffered with the set pieces and throw-ins so we knew we would struggle against them," he said.
"The players have to understand we cannot do this anymore, we know the context of the season and we need to change that."
Mason Mount scored his first goal of the season to put United ahead, but a Luke Shaw own goal and Schade's strike put Brentford ahead by halftime.
Schade got his second with a close-range header from Bryan Mbeumo's delivery, before Yoane Wissa bagged the fourth from six yards out as United's defence collapsed.
Alejandro Garnacho's 82nd-minute rocket from the edge of the area and Amad Diallo's stoppage-time effort put a little gloss on the scoreline for an outclassed United.
RARE BRIGHT SPOT
After two injury-plagued seasons since arriving from Chelsea, Mount's goal was a rare bright spot for Amorim.
"It's not just the finish, it's the position he has on the pitch, he's a third midfielder who can reach the box, he's really intelligent and he's getting there," he said.
"He trains really hard, sometimes too hard and I have to pull him back and take him off the pitch because he wants to do more.
"He wants to play more minutes and I'm really happy with him."