The western had its first hiccup when Fassbender left the film due to scheduling conflicts, with Law coming aboard in his place.
The film ran into more serious trouble when original director Lynne Ramsay ("We Need to Talk About Kevin") failed to show up on the first day of the shoot, leading to her replacement by "Warrior" director Gavin O’Connor.
While stars Natalie Portman and Joel Edgerton stayed on, Law -- who was playing the film's villain -- exited the project, leaving the bad guy role up for grabs.
Things looked dire indeed for the film, but now "Hangover" star Cooper is here to save the day.
Coming off an Oscar nomination for "Silver Linings Playbook," the busy actor has stepped into the vacant role two weeks into production, according to Deadline.
The move could end up not only saving the troubled production, but lifting the low-budget indie's box office potential.
"Jane" finds a woman (Portman) calling on an ex-lover (Edgerton) to help her protect her homestead when bandits (now led by Cooper) show up, looking for her troubled husband.
Cooper is currently starring Ryan Gosling in "The Place Beyond the Pines," and will next be seen in "The Hangover Part III." He'll also reunite with "Silver" director David O Russell for an as-yet-untitled film.