When Sammy Vasquez Jr. found out he would be facing Felix Diaz on July 16 instead of veteran Luis Collazo because of a calf injury, Vasquez’s blase’ reaction shouldn’t have come as a surprise.

A change of opponent a little more than two weeks before a nationally-televised fight on Fox? No problem. It just involved some slight recalibrations, a little mental gymnastics, which for someone who served two tours of duty in Iraq as a member of the national guard, wasn’t that much of an exercise.

“It’s a little change but nothing too big,” said Vasquez, whose match with Diaz will serve as the co-feature to Deontay Wilder’s heavyweight title defense against Chris Arreola from Birmingham, Alabama. “We always prepare for everything.”

Vasquez (21-0, 15 knockouts) has quickly earned a reputation as a rising star with an extraordinary back story. A day after July 4th, the 30-year-old spoke freely of his commitment to his country and of the burden he still carries from his time in Iraq.

Vasquez was diagnosed with post-traumatic stress disorder upon his return, so disturbed was he by what he saw there. He uses boxing as a stress reliever, to “slow things down,” as he puts it, so he can see things clearer. “Boxing is like therapy for me,” he said in a phone interview. “I can comprehend a lot more because I’m settled.”

To those who know him, Vasquez is calm and relaxed. But the cool exterior belies what lurks beneath. He was diagnosed with PTSD when he returned from deployments in 2005-2006 and 2008-2009. “That’s pretty much something that’s never going to go away,” Vasquez remarked. “That pretty much sticks with you no matter where you go. It’s just trying to deal with it better and better each time something comes up that triggers you.”

Vasquez relays combat stories of seeing friends killed in battle quickly and matter-of-factly, making the transition to a discussion of a boxing match seem ridiculous. He has steadily climbed the ranks since turning pro in 2012, his bouncy, aggressive southpaw style pleasing to fans.

A win over the respected Diaz would further stamp him as a contender in the loaded welterweight division. He has targeted WBC champion Danny Garcia for his title shot in an all-Keystone State matchup. “It’s a very marketable fight,” he said. “He’s from Philly and I’m from (near) Pittsburgh. I think it would be a huge draw between us. I’m a come-getter and he’s more of a counter-puncher. I just think it would be a great fight for Pennsylvania and for everyone else.”

Vasquez recently purchased a home on five acres of property in Colorado Springs, Colorado, where he now lives and trains. Junior welterweight titleholder Terence Crawford and Vasquez both prepare for bouts at the Triple Threat Gym there. Crawford and Vasquez haven’t sparred together for their upcoming fights – Crawford faces Viktor Postol in a junior welterweight unification match on July 23 – but they are workout companions and have developed a close bond. “I learn a lot from watching him,” Vasquez said. “We train together, shadow box together. I watch some of his workouts. I’ll watch him spar, he’ll watch me spar. We critique each other. He’s a really cool dude.”

Vasquez was diplomatic in discussing the switch of opponents from Collazo to Diaz (17-1, 8 KOs) and if Diaz, a former Olympic gold medalist, poses the tougher challenge. Though Diaz is regarded as the fresher of the two, he is also smaller and has fluctuated between 140 and 147 pounds while Collazo is a full-fledged welterweight. Diaz is a southpaw, like Collazo and Vasquez. Vasquez sounded as if he was viewing Collazo as the sterner test.

“I know I hit harder than most of the guys that he fought,” Vasquez said of Diaz. “I want to see what he brings once he gets hit. He is 5-5. I would assume that Collazo has more power than Felix Diaz does and (Collazo is) the same height as me (5-10) and Luis Collazo was elusive, too. He was a slick, southpaw, just like Felix Diaz. But now Felix Diaz is smaller and he’s a 140-pounder. He doesn’t have a lot of knockouts. But we’ll see what happens. I know it’s going to be an exciting fight.”

Mitch Abramson is a former reporter for the New York Daily News and can be reached on Twitter at: @Mabramson13.