The actor plays beaten Buchanan's opponent, Panamanian Robero Duran's trainer Ray Arcel in Hands of Stone which includes the story of the 1972 fight between the pair in Madison Square Gardens, New York.
The lengthy fight ended after 13 rounds, when Buchanan was punched below the belt, and has been brought back to life on a Los Angeles sound stage.
The shoot in January this year saw Duran's part taken by actor Edgar Ramiraz, and Buchanan's by Irish boxer now actor John Duddy.
Duddy said De Niro played an influential role in getting him the part after another actor pulled out. He said: "I was training and sparring with Bobby to get him ready for his role in Grudge Match.
"He was then going on to play Ray Arcel in Hands of Stone. The director had already lined up an Australian actor for the role of Ken, but when he pulled out at the last minute, Bobby suggested me for the role. I can't thank him enough.
"It's a huge honour to play Ken. He's a hero, one of the great fighters and I just hope I do him justice. The filming took quite a while, but it was a great buzz.
"The director had me come into the mockup of Madison Square Gardens acccompanied by bagpipes."
He said he tried to defend Buchanan's honour after fearing that shots of the film may have made it look like Duran's punch was an accident.
Edinburgh-born Buchanan, 68, said: "I'll certainly be watching it."