Joe Unitas told the Baltimore Sun Flacco would handle the football scenes in the shooting of "Unitas We Stand" this summer.
"The way Joe plays and his demeanor reminds me a lot of my dad," Unitas said. "He's not a rah-rah guy. He throws a touchdown and maybe there's a little fist pump."
Unitas called Flacco, who led the Ravens to a Super Bowl victory this winter, a "good role model for kids."
Flacco said in a written statement posted on the film's website he "was honored by the opportunity" and assured fans the movie would not interfere with his preparations for next season.
Johnny Unitas died of a heart attack in 2002.