He made three halftime substitutions, with Cristiano Ronaldo — back in United's team after an offseason when he pushed to leave the club — staying on and playing the full match. He wore a frustrated look throughout.
"What I asked them to do was play with belief and take responsibility for their performance," Ten Hag said. "That is what they didn't do."
Josh Dasilva, Mathias Jensen, Ben Mee and Bryan Mbeumo were the scorers for Brentford, who pressed relentlessly and blew United away.
"It felt like we were on top of everything," Jensen said. "They couldn't cope with our pressure, our second balls, our intensity. You could see they were struggling."
It might get even worse for Ten Hag and his new team, who opened with a 2-1 home loss to Brighton last weekend. Up next? It's only fierce rival Liverpool, who will be eager to hand United a fifth straight loss stretching back to last season.
"We are, as a team, in a difficult process," Ten Hag said. "Of course, you expect a different start. It is not what we expected. It seems like what happened in the past, last season, we bring into the new season. We have to change that really quickly."
United's meltdown came in the last of Sunday's seven games that were played in sweltering conditions that reached as high as 35 degrees Celsius.
Manchester City and Arsenal both scored four goals in home wins and continue to set a hot early-season pace.
Man City didn't even need Erling Haaland to score in their 4-0 thrashing of Bournemouth.
Ilkay Gundogan, Kevin De Bruyne and Phil Foden were City's scorers along with an own goal, with Haaland barely getting a sight on goal in his first match at Etihad Stadium.
The Norway striker scored both of City's goals in their opening-weekend 2-0 win at West Ham but was mostly crowded out a week later, making just eight touches before coming off in the 74th.
A player who left City is thriving at Arsenal.
Gabriel Jesus played a starring role in the 4-2 win over Leicester, scoring the first two before halftime and providing the assists for second-half strikes by Granit Xhaka and Gabriel Martinelli.
Jesus, the Brazil striker who left City in the summer for a reported $84 million, was in hot form for Arsenal in preseason and appears to be relishing being the starting striker once again after being in and out of City's team.
An own goal by William Saliba and James Maddison's strike brought Leicester twice to within one goal of Arsenal at Emirates Stadium.
Elsewhere, they were teammates for England, but Steven Gerrard and Frank Lampard were rivals in the dugouts as they went head-to-head for the first time as managers.
Gerrard came out on top. Despite a late wobble, Aston Villa beat Everton 2-1 for their first win of the season. Danny Ings and Emi Buendia scored for Villa before an 87th-minute own goal by Lucas Digne gave Everton hope.
Like Man United, Everton are on zero points after two games.
Back-to-back draws were probably as much as Fulham could have expected on their return to England's big time.
It could have been much more, though, with Aleksandar Mitrovic having a late penalty saved in a 0-0 draw at Wolverhampton.
Jose Sa dived to his right to claw away the spot kick from the Serbia international, who scored twice in the 2-2 with Liverpool last weekend and was looking to score his 45th league goal in as many appearances for Fulham.
There were two other draws, with Brighton and Newcastle toiling to a 0-0 stalemate and Southampton coming from 2-0 down to draw 2-2 with Leeds.
