The movie will be produced by Warner Bros and Cross Creek Pictures and is based on the book, 'Black Mass: Whitey Bulger, the FBI, and a Devil's Deal'.
Directed by Scott Cooper, the film is currently being shot in Boston now, said the Hollywood Reporter.
Pearce, 46, was assigned to play Billy Bulger, Whitey's brother but quit after a few weeks for uncited reasons. Cumberbatch, 37, was approached a few weeks earlier and the studios are vouching to seal a deal with him.