O'Connell was primed for the role, having grown up boxing, but Winkler, who also trained in the ring during his youth, was really impressed by how good he was - and as a result, the Brit's stunt double didn't have to do much work.
"Jack O'Connell could be a professional boxer. He's an unbelievable fighter and did all his own stunts," Winkler tells WENN.
"There was just one punch thrown by someone else. He's such a good fighter and moves his body in such a beautiful way... He's a stunning fighter and a stunning actor..."
"We didn't have to spend any time teaching him how to fight," the filmmaker continues. "We were trying to catch up to him if anything."
O'Connell and Winkler worked closely with the stunt co-ordinator, boxing coach, and other fight advisors on the set of "Jungleland" to ensure every move was fully rehearsed, and Jack nailed every shot.
"When you're doing a boxing movie with gloves you have the luxury of missing (a punch) and it's OK because getting hit in the face with a glove feels different than when you're doing a bare-knuckle boxing movie - you don't have that luxury because if you miss, you can knock someone out or break your hand," Winkler explains. "Jack is such a good fighter that he never missed."
"Jungleland", which Winkler co-wrote with David Branson Smith and Theodore Bressman, is out now in select theatres and on video-on-demand services.