The former England loose forward was stunned by Friday’s awful 63-12 home loss to Warrington Wolves.
It was the biggest defeat Hull had ever endured at the KCOM Stadium and highlighted just how far off the pace they are when it comes to being touted as Super League title contenders.
Westerman, 29, conceded: “We were just not at the races.
“It was embarrassing, really. Just embarrassing.
“Defensively we weren’t there; they rolled us and taught us a real lesson.
“It’s a tough one explaining what we put it down to as we trained well all week and we expect to turn up and put that training into the game.
“But it’s got to come down to an attitude thing really.
“It showed on that second set; they got their noses through and did a 100 metre set which is not good enough from us. Not good enough at all.”
His former team were 28-0 up inside just 21 minutes with four-try Australian stand-off Blake Austin given the freedom of KCOM Stadium.
“We made him look like the marquee player he is didn’t we?” said Westerman.
“We were terrible.”
It was a second wretched home display in a row after a woeful 32-12 loss to Wakefield Trinity.
In between, they had won at London Broncos and, admittedly, this was only a second defeat in their last six matches.
But Westerman said: “He (coach Lee Radford) is not happy.
“We have to address it immediately and try and get right for next Sunday.
“We must turn it around straight away at Salford as another loss and we’re struggling.”