The Dutchman watched his side thrashed 7-0 at Anfield after a meek display at the home of their bitter rivals.
Mohamed Salah, Darwin Nunez and Cody Gakpo all bagged braces before late substitute Roberto Firmino added a seventh in the dying embers of the game.
United capitulated in the second half after Gakpo’s opener had arguably come against the run of play before the break.
Liverpool would hit ten Hag’s side for six without response after the interval and could well have had more, such was their inability to clear lines.
Reacting angrily after the game, ten Hag questioned his players in the wake of the loss, branding them “unprofessional”.
"I am surprised because I have seen the last weeks and months this team is resilient and has a winning attitude,” he said.
“Second half we didn’t have a winning attitude at all. We didn’t stick to the plan and we didn’t do our jobs.
"We didn’t track back and it was really unprofessional. We have seen in the past we can bounce back.
“After Brentford, after Manchester City. This is definitely a strong setback and is unacceptable. I’m really disappointed and angry about it.”
Liverpool now sit seven points off United as Jurgen Klopp’s side attempt to track down the top four between now and the end of the season.