James DeGale’s trainer, Jim McDonnell, says the Harlesden fighter’s mentality and maturity has grown during his latest camp ahead of his huge unification clash against Badou Jack next month in New York.

The 30-year-old puts his IBF world super-middleweight title on the line when he takes on WBC champion Jack at the Barclays Center in Brooklyn on January 14.

And McDonnell, the man behind DeGale’s rapid rise to the top, has hailed the improvements his fighter has made in preparation for the toughest showdown of his career to date.

He told the Times: “When James boxed [Rogelio] Medina we trained hard and got the job done, but mentally we weren’t 100 per cent there.

“Physically he did all the things he needed to do to win that particular fight, but for this fight he’s got to have both of those factors, and he now does in abundance.

“The strength and conditioning coach we have brought in has made a major difference, too. He’s stronger and fitter than he’s ever been.”

McDonnell continued: “Don’t get me wrong, Badou Jack is a very good fighter, but he does the basics well and does nothing special, whereas James does everything well and has that x-factor.

“The first six rounds will be really competitive and I think in the second half of the fight you’ll see exactly why James is world champion and there’ll potentially be a late stoppage.

“This is the toughest fight of James’ career. He’s got a challenger’s mentality, and that’s the best way to be.

“He’s grown into being a champion and shown a lot of maturity – he’s really stretched the boundaries. He thrives on the pressure rather than the other way round.”

If DeGale is successful against Jack, McDonnell says fights against Callum Smith and arch-rival George Groves are likely to get the go-ahead in the new year.

“Callum Smith’s a good prospect and a talented fighter,” said McDonnell.

“He comes from a boxing family and has real potential, but I don’t see that as one which would trouble James too much if it went ahead.

“I think George has got a fantastic chance of winning a title now. (Fedor) Chudinov is a bit of a weak link, so I think he’s got a great chance of doing it. If he wants the rematch with James he’ll need to get that WBA super-middleweight title, and I’m sure he knows that.”