Trainers can improve many things about a fighter’s defence. They can teach a fighter to keep his hands up, tuck his chin in, move his head. But there hasn’t been a trainer yet who could teach a fighter to withstand a dynamite shot to the jaw.

Which is why the doubts over Amir Khan’s ability to step up to world class linger, and why some observers believe any amount of defensive fine tuning will count for little when he climbs into the ring with a top-drawer opponent who packs a wallop.

Colombia’s Breidis Prescott may not be from the top drawer, he may not even be from the middle drawer, but with 17 knockouts in 19 unbeaten encounters, he can hit. Which is why Khan’s promoter Frank Warren admits Saturday’s fight at Manchester’s MEN Arena is “a gamble”.

Jorge Rubio of Cuba will be in Khan’s corner for the first time, and it will be fascinating to see if he has brought about a significant change in the former Olympic silver medallist’s style.

"It's been like a switch back to the amateur days, back to basics, with a lot of speed work,” says Khan.

“Everything's gone so well I feel strong and fit and I've made the weight easier."

In the build-up to the fight, Rubio has spoken of the need for Khan to throw more body punches, to fight more on the retreat, to show greater caution. Insiders say Rubio is a gnarled operator who likes things done his way.

Khan sounded out top American trainers Floyd Mayweather Sr, Buddy McGirt and Freddie Roach before deciding the unheralded Rubio was the man to iron out his defensive failings.

"Toughest night yet"

Rubio, who coached the Cuban team at the 1996 Olympics and who has worked with former middleweight world champion Jermain Taylor and David Haye, was a left-field choice.

But the no-nonsense trainer from the Extreme Boxing Club in Miami might be just the medicine the wealthy young Khan needs at this stage of his career.

But back to Khan’s jaw. Willie Limond, a man who has stopped just eight of his 31 opponents, had Khan down last July. Michael Gomez, long past his best, had him down again in June. Which is why Prescott fancies his chances.

“Khan has a glass jaw and doesn’t hold a shot well at all,” says the 25-year-old from the tough port town of Barranquilla.

“He’s been floored by non-punchers so what chance does he have against me? I will knock him out with a left hook in the fourth.”

The 21-year-old Khan, undefeated in 18 paid contests, reckons it could be his toughest night yet.

“It’s very risky," says Khan. "It's a big step for me fighting an opponent with a big knockout percentage, he's undefeated just like myself and it's a 50-50 fight.

"He's going to be strong - he's got the height advantage, he's got the reach advantage, but I've done my homework and I know how to deal with that.

“He may have a good record but who has he fought? He's never fought anyone like Amir Khan."

A closer look at Prescott’s record reveals Khan is correct. Of Prescott’s 19 opponents, 12 had losing records and nine were winless.

In his last fight, and only his second outside of Colombia, Prescott sneaked a split decision against another inexperienced prospect in Cuba’s Richard Abril.

The Bolton fighter should have too much class for Prescott, but keep an eye on Khan’s knees: if Prescott can make them sag just once, the vultures will take flight.

Arthur should be too strong

Nicky Cook says the WBO super-featherweight title is still vacant, and while technically he’s wrong, you can see what he’s getting at.

Arthur was handed the title in May when previous incumbent Joan Guzman of the Dominican Republic declined a defence against the Edinburgh fighter and moved up a division.

And even though Arthur will carry the WBO belt to the ring on Saturday night, he will be acutely aware he still has a lot to prove.

Indeed, Cook could also argue that he has more experience fighting at genuine world level than Arthur, having lost a tilt at the WBO featherweight crown last July.

Arthur has certainly never boxed anyone as cute as Steve Luevano, who dropped Romford’s Cook five times with body shots and who has since defended his crown on three occasions.

And in his last fight, against Stephen Foster Jr last December, Arthur, who has lost just one of his 27 fights, was given a torrid time and suffered an 11th-round knockdown on his way to a unanimous points victory.

“Foster gave Arthur nightmares,” says the 28-year-old Cook, who has been defeated once in 29 fights. “And Foster’s only small.”

Cook, who will be conceding big height and reach advantages to Arthur in Manchester, is rightly encouraged by Foster’s performance.

What also gives him reason for confidence is Arthur and his trainer Wayne McCullough’s repeated talk of moving up to lightweight, and a possible fight against Khan.

“Arthur keeps talking about moving up to lightweight so I believe that’s the weight he believes he should be at,” reasons Cook.

“Him and Wayne keep talking about fighting Joel Casamayor, Manny Pacquiao or whoever, but they’ve got to do a job on Saturday first.”

Cook, who blames his defeat to Luevano on his struggle to boil down to the 9st featherweight limit, says he will be stronger carrying four extra pounds. And aware that this is probably his last world title shot, he will scrap tooth and nail.

The 30-year-old Arthur has long struggled with weight himself, but should be too big and too strong for Cook. And his body shots aren't bad either.