The psychological build-up to Anthony Joshua's prospective summer clash with Tyson Fury starts now, according to the man who masterminded one of heavyweight boxing's greatest upsets.
And McDonnell believes mind games could prove crucial in determining who has the edge between Joshua and Fury, after Joshua's promoter Eddie Hearn claimed the pair had signed a lucrative two-fight deal this week.
McDonnell told the PA news agency: "Everybody knows that physicality-wise there's nothing in it. But 80 per cent of the sport is in the mind, and that's why people make Tyson Fury favourite – if they gave points for press conferences, he'd win them all on points."
McDonnell ended Barry McGuigan's career and made two unsuccessful bids for the world super-featherweight crown, before becoming a successful trainer, first with Williams then in leading Olympic champion James Degale to a world title.
Picked as a patsy by Tyson for his latest comeback attempt, McDonnell employed a series of psychological devices in order to convince the Brixton no-hoper he had what it took to pull off his seismic fourth-round stoppage win.
McDonnell recalled: "Everyone had seen the way Danny had frozen in some of his previous fights and they didn't give him a prayer.
"But it was about getting inside his mind, creating this situation where when he walked to the ring he was so convinced he was going to win that fight that most of the job was already done.
"I offered sparring partners one thousand pounds of my own money if they could knock Danny down in the first two rounds. I wanted to recreate what it was going to be like when Tyson came out of his corner.
"When Danny beat Tyson it wasn't luck. You can come to the ring looking great and with your six-pack or whatever, but what really matters is the mind, and Danny was ready for anything."
McDonnell believes the ability of Joshua's camp to convince their man he has the beating of Fury could prove crucial in a contest for which opinion over the outcome is generally divided.
And he would not be surprised to see the Olympic gold medallist employing some uncharacteristic mind games of his own in a bid to wrong-foot Fury, who has a history of leading pre-fight build-ups from the front.
"Tyson Fury is king of the mind games, but if Joshua can get himself into a position where his mind is the same as someone like Floyd Mayweather when he is walking to the ring, then I make him favourite to win the fight," added McDonnell.
"Obviously in order to beat Tyson Fury you need great ability and you've got to be super-fit, but I think you've also got to take the build-up to another level.
"What you want with Fury is the personal side to come out, something that turns him from the ring genius into the guy who'll meet Joshua in the centre of the ring and make it personal.
"Fury is so good at the mind games, he's up there with the likes of Muhammad Ali. That's what makes him so hard to beat – but it's also what Joshua has to work on so that he will go into this fight with the advantage."