Adam Booth, the trainer of David Haye, says Dereck Chisora blew any chance he had of beating Vitali Klitschko by slapping him at Friday's weigh-in.
Britain's Chisora, 28, challenges WBC heavyweight champion Klitschko in Munich on Saturday.
"He's blown his only chance, the last thing you want to do is pick a fight with Vitali," Booth told BBC Sport.
"A cheap shot like that hasn't got a prayer of working against someone like Vitali. That was foolish."
Chisora caused mayhem at the weigh-in at a Munich sports shop when he slapped his Ukrainian rival round the face during the traditional stare-down.
The 40-year-old Klitschko, whose last defeat was against Britain's Lennox Lewis in 2003, reacted with embarrassment and anger, which Booth said would translate into much pain for Chisora at the Olympiahalle.
"Of course Vitali's embarrassed by it and he will turn that into anger in the fight," added Booth, who was in Haye's corner when he fought and lost to Vitali's younger brother Wladimir last July.
"There were rumours Vitali would carry him for a few rounds but now he'll be thinking 'never mind that, I'm chinning him'.
"If you're going to take a cheap shot against someone, do it to someone it might work against psychologically."
Booth added that he did not expect the fight to last more than three rounds, with the champion's power - he has 40 knockouts from 43 professional wins - proving the decisive factor.
"Vitali is an older man so you want to take him further into the fight where fatigue may play a part. But don't motivate him to set about you early," he said.
"David wasn't aggressive enough against Wladimir but the style for Vitali is totally different; Wladimir has a much better repertoire, is harder to hit but easier to hurt. Vitali is easier to hit but harder to hurt.
"Both of them are beatable but this is coming from a trainer who failed to prove his point with his tactics [against Wladimir]. We got half of it right but David wasn't aggressive enough.
"Vitali fights with intensity and he doesn't take a backward step unless that's the sensible thing to do. He wants to stand there and hurt you. And you've got a better chance at hurting him with a slap than a punch.
"There is a bad bunch of heavyweights at the moment but Vitali is the best of them. And even if the bunch was good, he'd still be up there."
Former British champion Chisora has 15 wins from 17 fights with nine knockouts while Klitschko has 43 wins from 45 fights, although one of his defeats was the result of an injury retirement.
Chisora has lost two of his last three fights, against Finland's Robert Helenius and fellow Brit Tyson Fury, and is a huge underdog for Saturday's fight.