Keiron Cunningham had little sympathy for sin-binned Mark Percival and was highly critical of his team after their first defeat in five matches.
Saints, who went top after beating Wigan a week ago, looked to have claimed the spoils when tries from Shannon McDonnell, Alex Walmsley and comeback duo Luke Walsh and Joe Greenwood gave them a 24-22 lead.
But they finished the game with 12 men following the sin-binning of centre Mark Percival for holding down in the tackle and also lost full-back McDonnell and winger Adam Swift to injury in the closing stages.
“You can’t do that,” Cunningham said of Percival. “It was just a stupid thing by the kid.
“The game probably went away from us when we pulled a shirt on before we took the field. We did our best to lose that game from minute one.
“It’s hard to be critical because they’ve been phenomenal over the last few weeks but you’ve got to be good every week in this competition.
“I thought both sides had a hangover from big games last week. We were awful in the first half and Castleford were just as bad.
“In the second half Castleford decided to have a bit more want and a bit more vigour about them.
“I think mentally the boys came with a soft attitude.
“Cas are a very good side, a top four side for a reason.
“We didn’t deserve to win it, we got what we deserved. If we had won, would have been a terrible injustice to Castleford. We’ve just got to improve.”
The game was held up for almost 10 minutes to allow Swift to receive treatment for a head injury and, after being carried off on a stretcher, he was taken to hospital.
“I think he just got caught awkward in the tackle,” Cunningham said. “It’s more precautionary than anything else and fingers crossed Adam’s okay.”
Already without winger Tom Makinson with a broken leg and first-choice full-back Jonny Lomax with a season-ending knee injury, St Helens are fearing the worst for McDonnell after he went down with what appears to be an Achilles injury.
“It’s just the way our luck is going at the minute,” Cunningham added. “It’s a shame for Shannon because he’s filled a great gap for us and he was very good again tonight. It’s unfortunate for the kid.”
Castleford, meanwhile, coach Daryl Powell would have settled for a draw but could not hide his delight after watching his side snatch another famous victory to strengthen their Super League top-four hopes.
A drop goal by full-back Ben Roberts in the last act of a pulsating match clinched a 25-24 victory for the Tigers, who celebrated a first win over leaders and champions St Helens for eight years, a week after securing a first away win over previous leaders Leeds for five years.
“It was an incredible finish,” Powell said. “Obviously we’re pleased to be on the right end of that.
“They got us here last year when we thought we should have won the game.
“For us belief is just growing all the time. You’ve got to be so pleased for the boys and the way they’re going about their business at the moment.
“We’ve got some confidence about us and we’re pragmatic at the right times as well. We’ve got a good balance at the moment.
“We’re hanging in battles and finding a way to win. We’re delighted with the way we’re going.”
Justin Carney marked his comeback from a dislocated elbow with the game’s first try and fellow winger Denny Solomona provided the highlight with a 95-metre sprint for his eighth touchdown in four games, but Powell was especially pleased with the contribution of Roberts.
The former Melbourne stand-off experienced a slow start to his Super League career but has revelled in a switch to full-back and demonstrated his kicking ability with his snap drop goal.
“A draw would have been fine but we put a message out there and he struck it pretty sweet,” Powell said. “It was pretty smart from him.
“He had a tough time at the back with their kicking game but he did some really good things and he’s becoming a real good player for us.”