When Amir Khan tests himself Saturday night against three-division world champion Marco Antonio Barrera, it will not be a physical test. It will be an intelligence test.

Younger, faster, taller and hungrier, England’s 2004 Olympic silver medalist will have everything going for him, including home field advantage, when he faces Barrera at the M.E.N Arena in Manchester, England, just down the road from his native Bolton. In front of him will stand a shadow of who Barrera once was. Though still dangerous, Barrera is well past his prime, having begun the long, slow slide all great fighters face.

Khan’s promoter, Frank Warren, is a wise and wary man, someone not prone to risking his moneymakers against dangerous, stern opposition without being forced to. That he sought out this fight and agreed to a deal with Barrera’s promoter and his former partner, Don King, only made clear his faith that Barrera is no longer what he once was, because if he was, Amir Khan wouldn’t be in the same area code as Marco Antonio Barrera Saturday night.

Yet this fight remains a test all the same because like the greatest of Shakespearean characters, Amir Khan has a fatal flaw – his chin. While he is otherwise well equipped for the profession he has chosen, his chin has cracked on several occasions, most notably in the only loss of his career in Sept. 2008 when he was badly wobbled within seconds by a jab from Breidis Prescott, and soon after had his mind numbed to the passage of time until he was counted out.

At 35, Barrera is not as fast as he once was, or as resilient as he once was or as powerful as he once was… but he has retained all his fighting intellect, and that is the weapon Khan must overcome first. For if he cannot, all his other advantages could well dissolve as quickly as his chin sometimes has.

“This fight is going to be more about patience and it’s going to be more technical,’’ Khan (19-1, 15 KOs) claimed several days ago after returning to England following six weeks of training at Freddie Roach’s Wild Card Gym in Hollywood.

“The hands are going to be up. I’m going to be more focused. It’s going to be a clinical fight instead of me rushing in with my hands down, chin up. It’s going to be the total opposite. You’re going to see a new Amir Khan in this fight. I think it should flow well.’’

Roach has been working diligently on Khan’s defense, or lack thereof. He has his chin down, his hands up and his mind beginning to grasp the idea that he is not the only lightweight in boxing that can do damage when he lands. But Roach is the first to admit that sticking to your knitting is a lot easier in the gym than when the TV lights come up and the crowd is roaring and a formidable man is across from you trying to lash your features.

Only then will anyone know if there is indeed a new Amir Khan. Until that moment passes, all that is known for sure is that there needs to be a new Khan, because the one Prescott knocked stiff and several other lesser lights knocked down is not someone destined to become lightweight champion. In fact, that Khan might be destined to have his career ended by the aging Barrera rather than the other way around.

If Khan can keep that in mind for 12 rounds and not be lured into a firefight with an old warrior whose only chance is to take advantage of bad habits and youthful indiscretions then Roach’s work will have been rewarded. Certainly Roach has impressed upon Khan that while Barrera is not what he was a few years ago that does not mean he can be disrespected, especially early in the fight. Khan would be wise to be wary.

The problem most likely will come, if it does at all, after a few rounds of beating Barrera to the punch and eluding him with his speed: Will Khan still stay wary then?

That is the great unknown, but certainly there will be someone in his corner who can speak to him of the wisdom of that in the person of former heavyweight champion Michael Moorer, who has begun to assist Roach as a trainer and been what Khan calls “the third eye’’ in his training camp. Moorer has been watching for any slippage in technique during the 90 rounds of sparring Khan engaged in, some of it with Manny Pacquiao, who twice defeated Barrera when he was far more formidable than he is today.

Moorer also learned the hard way the dangerous side old veterans with proven pedigree bring with them to a boxing match. It was Moorer who was winning easily against George Foreman when he let down his guard for an instant and was knocked unconscious by a punch that traveled about six inches. The heavyweight crown fell off his head before his head hit the floor.

Moorer learned from that 1994 night and has tried to pass on the wisdom of caution without panic to young Khan, but only when the moment comes will Moorer know if all learning comes from doing or if, sometimes, wise counsel can be heeded after the fireworks start.

“I’m moving into world class level,’’ Khan said of Barrera (65-6, 42 KOs), repeating by rote the things he’s been told the past six weeks in L.A..

“You’ve got to think. You’ve got to pace yourself. You have to think about every move you make. Instead of using my heart, I have to use my brain a bit more.

“With this guy you have to box him. You have to be smart. I think if I just box him and do what I’m good to do and not get involved in his game and his fight then we’ll be okay in the fight.

“You won’t see the old mistakes I used to make. It’s going to be a professional job. You’re going to see a new Amir Khan.’’

Khan may be right about that, but that is also the question. Can he avoid getting involved in a fight when Barrera wants one to break out? Is there truly a new Amir Khan or an old one waiting to re-appear at the worst of moments?

Roach and Moorer have certainly worked hard to avoid the latter, making Khan more focused on defense with the understanding his offense must flow from that. Pacquiao has also counseled him against feeling he has to prove his mettle by matching one of boxing’s most revered warriors every aggressive act with one of his own.

It all sounded great. Made perfect sense, really, until Khan began to talk about how he would be there to prove his critics wrong and that his performance was “going to shut a lot of people up.’’ Khan’s previously ignored fighting flaws were loudly pointed out after the Prescott defeat at a time when he had grown used to being coddled by the press as well as the people around him. That is why Roach was brought on board, to not only train him but also to give him a shot of boxing’s harsh realities. But if he feels he has something to prove to his critics, he is wasting his energy and his emotion on things that will be of no value once the first bell rings.

Khan has the benefit of having one of the best trainers in boxing in his corner. He has a well-thought out fight plan by someone who has beaten Barrera twice with Pacquiao and of having gained an understanding that fools rush in where wise men fear to tread from Prescott.

Yet Roach knows well that once the fight starts odd things can happen. Amnesia can set in. Old habits can return. A fighter can go deaf to the counsel of his corner. Chins can go up and fighters can fall down.

That is particularly dangerous for a fighter with a suspect chin who felt the need to say, “I think everyone knows Amir Khan is a warrior. I’m game for a fight every time there’s a fight. But for this fight I’m going to be pacing myself.’’

That would be a wise choice. It would be the choice a professional would make. It is also the one Amir Khan best stick to because if a fight breaks out in Manchester instead of a boxing match don’t bet against the old Mexican.