Goody Petronelli, the only trainer Marvelous Marvin Hagler ever knew, always used to say, “You can’t train a chin.’’ That is what Freddie Roach has to be concerned about.

Roach has been working feverishly to prepare the popular British lightweight with the mangled mandible, Amir Khan, for Saturday’s showdown with the fading Marco Antonio Barrera at the MEN Arena in Manchester. Roach is a master craftsman, perhaps the finest trainer working today, but there are limits for any craftsman if his tool kit is limited.

Roach told an old friend a while back that “Goody was right,’’ when asked about Petronelli’s philosophy. No amount of weight work to strengthen neck muscles, for example, will alter how quick a fighter’s brain goes blank when hit on the chin. That ability to absorb shock is a gift from a Higher Power not something you can improve, like a jab or a hook.

What can be improved upon is what Roach has been emphasizing with Khan in the six weeks he had him at the Wild Card Gym in Hollywood. That is defense, keeping your chin tucked down and avoiding wild offensive charges that open a boxer up to being countered flush on the jaw.

Khan claims to have learned much during his six-week tutorial in the manly art of self-defense and surely he has. But has he learned enough to avoid making the kind of error Barrera will be looking for all night? Can he avoid both the mistakes of bold youth and also the mistakes caused by panic if things do not go well for a time?

Roach admits he has no more assurances of that than anyone else, although he believes Khan has the youth, hand speed, movement and height to baffle and batter Barrera all night long without putting himself in harms’ way. That, of course, is why the fight was made in the first place. British promoter Frank Warren agreed to the bout because he believes Barrera is an empty shell, a legend who will bring only his legend to the arena Saturday night.

He had best hope he is right about the 35-year old former world champion at 122, 126 and 130 pounds because if he is not, Amir Khan’s days as a big draw in England will have ended well ahead of schedule.

Barrera certainly looked to have slipped badly in his loss to Manny Pacquiao in his last major fight and bouts against two journeymen that followed did nothing to change that opinion, even though he won. In the second, a foolhardy tune-up in late January, Barrera sustained a significant cut from a headbutt but claims it will not affect him against Khan. Perhaps not, but Julio Cesar Chavez thought the same in his first fight with Oscar De La hoya after being cut in training and he was split wide open in a few short rounds.

That is not to compare Khan to De La Hoya, which would be insane, or Chavez to Barrera. It is just to recall that because someone says a cut or injury will not be a problem does not make it so.

Then again, same is true for a young fighter when he insists he is a new man. Wiser from being knocked cold by Breidis Prescott two fights ago, Khan says it was a blessing in disguise to have been wobbled by a stiff jab and then knocked cold in barely a minute because it forced him to admit changes needed to be made in his style and his training staff.

That was so, and changes have been faithfully executed, but none of that will strengthen his chin if Barrera lands a combination on it. If that happens and Khan wobbles again as he consistently has in the past when struck flush by a solid job or two or three-punch combination, all the training in the world isn’t likely to help him because the messages to his brain will be blocked and Barrera’s punches will not be.

The problem for Barrera is to do that he must turn back the clock. Not all the way back to his days beating back the challenge of Erik Morales or Khan’s idol, Naseem Hamed (who Barrera retired eight years ago with a one-sided beating of that equally popular former British star) but at least back to that loss to Marquez.

That night Barrera did not deserve the decision and he didn’t get it but it was not a one-sided fight even though Marquez is arguably second only to Pacquiao on the pound-for-pound list. He was not the Barrera who became a legend himself in Mexico but he also was not an empty shell. He was what Warren and Roach have to hope he will not still be two years later – which was competitive with one of the best boxers in the world.

History and his own sadder performance against Pacquiao later in 2007, a loss that led to his temporary (aren’t they all?) retirement before joining forces with Don King, would give anyone reason to doubt if Barrera has that one last epic performance in him.

In the past two years Kelly Pavlik, Antonio Margarito and Juan Diaz were all beaten by older and wiser men. Each one of those fighters was better and more experienced than Khan, who essentially has fought no one and is still 19-1. The one says more about him than the 19 but the question then remains, is this the night his chin betrays him a second and final time, or not?

Many have gone broke betting against youth and speed in boxing. To pick Marco Antonio Barrera over Amir Khan is to walk into that dangerous fire once again because if Khan can simply use that speed of hand and foot wisely Barrera may never get close to him.

That is the smart bet so if my children’s college funds (or what’s left of them) had to be wagered the money would be on Khan. But the funds do not have to be wagered, which is why I will side with Goody Petronelli this time and say you can’t train a chin.