The Tigers took an unlikely lead over the promotion chasers when Kamil Grosicki found the net with half an hour played, but United went on to claim all three points courtesy of four second-half goals from Leon Clarke.
The result made it three straight losses in the league for Hull, who are now just four points above the Championship dropzone, and Slutsky admitted afterwards that he was puzzled by his side's reaction to going ahead early on.
"I know what happened in each match and in the last three matches we stop playing after the goal," he told reporters. "We keep stopping to play after conceding. We don't have a strong mentality and team spirit. Each goal changes the quality of the game. It's like two teams play before the goal and after the goal. I don't have the answer in this situation.
"We were broken after the first goal. We must have faith in this situation but it is not the first time we lost it. It is my job I agree - if we don't stabilise the problem it is my fault.
"We changed the system, we changed the formation but it is difficult to change the process in between the games. We can't change the recruitment now, we can change only the reaction to each goal, it was good quality before the goal but broken after it.
"We have a lot of new players and I think maybe we don't have the team targets and maybe it is more individual targets. Everyone trains hard but on the pitch we played like mannequins. I have a problem with mentality and team spirit."
Next up for Slutsky's charges is a home encounter with Ipswich Town when EFL action resumes on November 18.