The 34-year-old actor, who plays a boxer and grieving father to 13-year-old Laurence, hopes his character's personal life one day becomes a reality for him, reported Female First.
"I would love it not to be fictional," he said.
The "Nightcrawler" star also revealed his fondness for his sister Maggie Gyllenhaal's children, and how they inspired his work on the film.
"I have two nieces whom I adore and whom I'm connected to in so many powerful ways. They are deeply engrained in the movie, and my love for them is there.