The Tigers were beaten 4-2 at the KC Stadium and in doing so missed an opportunity to move level on points with sixth-placed Nottingham Forest.
Jay Simpson had put them in front after just three minutes, but Boro stunned the home side with Scott McDonald scoring a first half hat-trick and Julio Arca also on target.
Anthony Gerrard pulled a goal back in the second half but the damage had already been done and Pearson was at a loss to explain his side's poor defending.
He said: "I don't think you can legislate for a first half like that. Sometimes you get days like today and it's a massive disappointment because it's a missed opportunity.
"Three wins might not be enough but there are nine points to play for and I won't tolerate anyone sulking.
"It's been an expensive day for us in terms of losing players through injury as well as the result. I don't think in the whole 90 minutes they created anything really apart from the goals, and we put those on a plate for them."
Source: PA
Source: PA