But when weakness turned into pain, he made an appointment with his doctor, believing he had a pinched nerve. An MRI exposed something much more severe -- there was a tumor on Jacobs' spine. Doctors diagnosed him with osteosarcoma, a type of bone cancer.
According to Jacobs, the tumor "chipped" his spine and ultimately left him partially paralyzed from the waist down.
"I couldn't walk for a good month and a half," the boxer recalls. "It was a very hard time in my life."
"Slowly but surely I saw a lot of people exit my inner circle," Jacobs recalls.
But as others began to walk away, his trainer Andre Rozier, remained in his corner.
"It was a very, very scary moment in our lives," Rozier says.
The fight of Jacobs' life
Doctors advised Jacobs not to enter the ring again, but he refused to give up on pursuing his dreams. He underwent surgery and 25 rounds of radiation which kept him from boxing for two years. Once healthy enough, his first goal was to learn how to walk again.
"We didn't know if he was going to make it initially," says Rozier.
So the coach took Jacobs' journey back into the ring in stages.
"There were new plateaus each time," Rozier remembers. "Okay, we can walk. We can jog, we can hit the heavy bag -- until that one moment he was able to perform like he did before. Any time he felt like doing more than he possibly could, I pushed him."
Jacobs beat cancer and defied the odds by going on to become the World Boxing Association's world middleweight champion.
"This was the hardest challenge in my life and I couldn't do it alone," Jacobs says. "Andre supported me through that time 100% ... I love him to death."
Rozier never questioned Jacobs becoming a world champion. In fact, he says he expected it.
"His battle with cancer was truly the hardest battle he could ever face. Everything else was a walk in the park."
A father and son relationship
For Rozier boxing is a family experience.
"I have a very special kinship with all my athletes," he says. "We have a very family-oriented environment."
With Jacobs, the veteran coach feels their relationship is best described as father and son.
"He taught me a lot about how to be man," Jacobs says. "He means the world to me."
Rozier started coaching Jacobs when the fighter was just 14 years old and was immediately impressed with his talent.
"I saw the changes in maturity and his growth. I said, 'He is going to be a really good fighter,'" the cornerman recalls.
Rozier's love of boxing began inside the ring. At 11 years old, he boxed as an amateur in the junior Olympics. At 16, he competed in the Golden Gloves. When high blood pressure forced him to hang up his gloves, he thought his dreams were dashed.
But a "new fire was awakened" in Rozier when a young man from his Brooklyn neighborhood asked Rozier to coach him.
"His confidence and exuberance brought me back," Rozier says. "This time as a teacher, instead of one being taught."
The job of a great cornerman
Known for his bright smile and calm demeanor Rozier's training style is no-nonsense -- he jokingly describes it as sadistic. His athletes would say he is "unsympathetic."
"I don't cater to mistakes," the 52-year-old laughs. "I don't like to lose."
The main reason behind his toughness? He doesn't want his boxers to experience fatigue in the ring. Rozier says the job of any great cornerman hinges on three essential responsibilities: to give guidance, correct mistakes and lift a fighter through a bout.
"The key thing about Andre's training style is his passion," Jacobs says. "It is his will to win that he shows and displays."
But for Rozier, it's his genuine love for his athletes and the sport that make the difference.
"I live, sleep, drink, this sport of boxing," he says.
"We are going to work hard. We are going to rise to the occasion ... I love my guys so it's easy for that to happen."