Jude Law, in the meantime, has signed on for the villain, sources tell The Hollywood Reporter.
The moves come a couple weeks before the start of shooting in New Mexico. Sources say the actor rotation came about due to Fassbender's schedule conflict with his upcoming shoot in Montreal for X-Men: Days of Future Past. Gun has been pushed back several times and now is scheduled to begin filming in April.
READ MORE: http://www.hollywoodreporter.com/heat-vision/michael-fassbender-exits-natalie-portmans-427402