UNCASVILLE, Connecticut – Derrick James didn’t dismiss Jeison Rosario’s career-changing accomplishment.

Jermell Charlo’s trainer just considers Rosario’s upset of Julian Williams more a reflection of what Williams didn’t do than what Rosario did. Williams was a 30-1 favorite when Rosario hurt Williams and stopped him in the fifth round January 18 at Temple University’s Liacouras Center in Philadelphia, Williams’ hometown.

Rosario’s surprising victory earned him a 154-pound title unification fight Saturday night against Houston’s Charlo, the WBC super welterweight champion.

“I don’t think it was a fluke,” James told BoxingScene.com. “I think it was a phenomenal performance from Rosario, but I think it was more J-Rock than it was him. I don’t think J-Rock had a bad day, but he was distracted. His focus wasn’t there. Maybe [Rosario] will prove me wrong, or do his best to, but it was more J-Rock, I think, than Rosario. I believe Rosario is a tough, rugged fighter, but I think we never got to see the best of him in that fight because J-Rock was distracted, I believe.”

The rugged Rosario rocked Williams with a left hook with just over two minutes to go in the fifth round. Williams (27-2-1, 16 KOs, 1 NC) tried to hold to survive, but Rosario drilled him with a right uppercut and a left hook that left Williams slumped against the ropes, unfit to continue.

Referee Benjy Esteves stopped their scheduled 12-rounder at 1:37 of the fifth round.

“Rosario poses a great threat,” James said. “He’s a big puncher. We’ll have to be focused throughout the whole fight. We can’t be relaxed, can’t get comfortable. And that’s what being a champion is, never being able to be comfortable anyway. That’s when you lose a step. The guy who Rosario is will keep us on our toes, and you’ll see a phenomenal boxer in Jermell Charlo – a very intelligent, very intellectual fight.”

James expects Charlo-Rosario to become “the war” Rosario has predicted at times. He also is confident Charlo’s boxing ability and ring IQ will help him withstand Rosario’s aggression and leave the ring with three titles.

“I think he will remind people of the boxing ability of Jermell Charlo,” James said. “I think people forgot about it because he went on a knockout streak. If you look at the way he’s been challenged throughout his career, he’s been able to endure. Vanes [Martirosyan] is a big puncher. Tony Harrison’s a pretty good puncher. [Charles] Hatley’s a good puncher. [Erickson] Lubin’s a good puncher. What you’ll see is all these fights prepared him for this fight right here. You’ll get to see everything. … I think you’ll see the best of Jermell Charlo, because in camp he’s been phenomenal.”

Charlo (33-1, 17 KOs) and Rosario (20-1-1, 14 KOs) will headline the second portion of Showtime Pay-Per-View’s six-fight card at Mohegan Sun Arena. They’ll fight for Charlo’s WBC and Rosario’s IBF and WBA titles.

The show, which will start at 7 p.m. ET/4 p.m. PT and costs $74.95, also will include Jermall Charlo (30-0, 22 KOs), Jermell’s twin brother, and Ukraine’s Sergiy Derevyanchenko (13-2, 10 KOs) in a 12-round fight for Charlo’s WBC middleweight title.

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.