LAS VEGAS — He wore loose-fitting clothes, and when it’s Julio Cesar Chavez Jr. involved, one always must worry, particularly when he must hit a specific number on the scale in a few days.

But when Chavez made his way into the MGM Grand the other day, he looked lean and thin, ready to compete in a big fight. His loose-fitting clothes seemed more of a fashion choice and not a means to conceal something like a flabby belly.

Chavez being who he is, though, there are always doubts. Guys like Tony Mandarich, JaMarcus Russell and Ryan Leaf, among the biggest NFL draft busts ever, come to mind when thinking of Chavez.

Like Chavez, they had the measurables but not whatever it is that it takes to succeed at the highest level of professional sports.

For most of his career, Chavez hasn’t been interested in the Spartan life it takes to be an elite boxer. He was talented enough to beat most guys he faced even when he only gave a half-hearted effort.

He’s not a bad guy by any stretch; on the contrary, he’s an engaging and personable young man who is enjoyable to chat with and who is fun to be around.

He’s been a good-time Charlie in a sport that harshly penalizes that type of personality. And so, as he gets a second crack at a career-defining bout, Chavez acts as if he understands all it entails to be a professional and to defeat Canelo Alvarez on Saturday at T-Mobile Arena.

[Related: Why Alvarez vs. Chavez Jr. could be dangerous for De La Hoya]

To this point in their careers, Alvarez has been everything that Chavez is not: Alvarez not only puts the work in, he goes the extra mile. Alvarez always has time to watch film or practice his technique. He’s not only ready to awaken early and do his road work, he’s eager.

His success is built from the sweat of his brow, even from his earliest days when he proved to be unusually determined and dedicated.

Chavez, though, is a different story. He has a wonderful record – he’s 50-2-1, and his only losses were to Sergio Martinez, who is likely to wind up in the Hall of Fame as soon as he becomes eligible, and light heavyweight Andrzej Fonfara.

Saturday’s bout is being fought at a catch weight of 164.5 pounds, and there was so much concern among promoters whether Chavez would make it that there reportedly was a penalty written into the contract that he would have to pay $1 million for every pound he came in over in Friday’s weigh in.

It’s hard to remember that when he began his career nearly 14 years ago, he weighed in for his pro debut on Sept. 26, 2003, against Jonathan Hernandez at 130 pounds.

In his 54 pro fights to this point – he had a no-contest in 2009 when he failed a post-fight drug test – Chavez has been under 140 pounds 16 times and under 150 pounds 30 times.

He matured into a quality middleweight and scored impressive wins over the likes of Andy Lee, Marco Antonio Rubio and Sebastian Zbik, among others.

But he didn’t have the discipline to take that next step toward greatness. He was never in Martinez’s class as a boxer, but when they met in 2012, Chavez’s punching power was an advantage, particularly against a slowing veteran.

The stories of him missing training during that fight are legend by now, and it makes one wonder whether his mistakes were a rich kid, son of a legend, not knowing what it is to sacrifice or something more.

It very well could be that he didn’t believe fully in himself, and he goofed off and avoided training as a way of providing himself an excuse.

Despite everything, he was only a punch away from winning the Martinez fight in the 12th, and the great Martinez had to hang on to survive.

Now, Chavez says he trained hard for that fight, but wasn’t mentally prepared.

“I wasn’t focused and didn’t do everything I was supposed to do,” he said, which was hardly a revelation to anyone who paid attention. “I wasn’t focused enough and I let a lot of rounds go by.”

If he comes in like that against Alvarez, he’ll get his block knocked off. Alvarez doesn’t fool around, and he’ll hurt Chavez badly if, as he’s done so often in the past, Chavez has lollygagged his way through camp.

[Related: Canelo Alvarez gives his side on Triple-G/WBC fight debacle]

Chavez insists this is different, and of course he does. It’s easy to portray yourself as changed, that you’ve seen the error of your ways and that now things are different.

When it’s Chavez, believe that at your own peril.

But, he does look different. The pictures of him from his training camp showed a six-pack that didn’t come from having a couple of Krispy Kremes with his coffee for breakfast.

He hired the legendary Nacho Beristain to train him. Beristain has been one of the sport’s elite trainers, and is rightly in the International Boxing Hall of Fame. While he’s known for helping fighters to improve their techniques and is a master strategist, he’s also a renowned disciplinarian.

At this stage of his life, in his 78th year, Beristain doesn’t need to put up with a chronic underachiever and a guy who isn’t serious about being as prepared as possible.

Beristain is as straight of a shooter as they come, and he insists Chavez has done what is needed.

“We’re already close to the weight,” Beristain said Tuesday, three days before Chavez would have to face the music and hit 164.5 or lose a lot of money and a lot of pride.

He found Chavez to be extremely talented.

“He has so much ability and I’m very surprised at his capacity,” Beristain said.

After you’ve been fooled multiple times by a con artist, it’s never going to be easy to believe anything he says.

That clearly is the case with Chavez now.

He’s passing the eye test, though, and he seems to have convinced Beristain he’s serious.

If Beristain is correct, and this is not more smoke and mirrors from Chavez, Saturday’s bout is going to be much more interesting than many had previously believed.

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