The gutted coach said he was surprised supporters stuck around so long on Sunday afternoon as the Tigers slumped to their second-biggest loss as a joint venture.
They were never in the contest against the ladder-leading Panthers, who scored five of their six first-half tries on the back of errors from their rivals.
Asked what he told his players in his halftime address, Marshall was blunt.
"You don't want to know," he said.
The joint venture did not play the ball in the red zone until the second half, conceded 15 line breaks and missed 55 tackles for the afternoon.
Among their more embarrassing defensive moments, Penrith scored twice from dummy half when Freddy Lussick flopped past the defence.
Co-captain Jarome Luai also threw an offload straight to Liam Martin in the red zone ahead of an easy first-half try for the Panthers.
"I don't usually apologise to our fans and members but that performance was just unacceptable," Marshall said.
"I'm actually surprised they stuck around as long as they did today, because it was clear to me from the start, we weren't there.Â
"Just fundamentals of the game. It looks like with the scoreline, we're not trying. We're so dumb with the ball, and we can't defend our errors, we're tackling so much, then we get brain-dead and we can't compete. It's disappointing, and actually embarrassing."
Marshall said it was a "real concern" that the Tigers had lost by at least 28 points in four of their past five games, following an improved start to the year.
"Last week we turned up with a real defensive resolve and that was non-existent today," he said.
The Tigers are missing key men Alex Twal, Adam Doueihi and Taylan May to injury, with Jock Madden (elbow) and Royce Hunt (pectoral) joining the casualty ward against Penrith.
Starford To'a, Luke Laulilii, Heath Mason and Tony Sukkar are among the first-graders currently out of the team, with Marshall saying he would consider making changes
"We'll have a look," he said.
"You have to take accountability for your performance.
"If you flush it, then you're saying that what happened today, we're prepared to just put under the rug and accept it's OK. It's not."
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