Nathan Brown admitted that St Helens produced the poorest display since his arrival in the 24-12 defeat by Hull KR.
Rovers raced into a 16-0 lead with three tries in the opening 20 minutes from Travis Burns, Alex Brown and Greg Eden.
Saints replied with second-half touchdowns from Francis Meli and Anthony Laffranchi, but they could not prevent their first home loss against the Robins since 2009.
"I can't fault our effort but we never got in the contest and it was our most disappointing performance since I've been here," said Brown.
"Our attack was poor. They kept kicking the ball behind our wingers and we did it once.
"They kept forcing us into errors and we weren't smart.
"It was disappointing for the fans because we are capable of better than that.
"We had been building along nicely recently but we have taken a step backwards.
"I didn't think their first try was a try but we didn't respond to that."
Rovers coach Craig Sandercock was delighted with his in-form side after they picked up their fifth win in the last six games.
"It was a big win and gives the boys a belief they can win away from Craven Park," he said.
"We were outstanding defensively.
"We conceded too many penalties for my liking but we showed great defensive resolve.
"Usually our attack is not too bad but it's our defence that gave me the most pleasure.
"They gifted us some possession but our defensive effort forced them into errors.
"We won't get carried away because we are still in the bottom half of the top eight.
"There is a lot we need to improve but we have been under the pump at times this year and I can't speak highly enough of the boys."