Magnussen, who will be seen in the Disney film adaptation of Into the Woods, would replace Charlie Hunnam, who was slated to star as elusive billionaire Christian Grey; Hunnam recently withdrew from the film after being denied extensive script approval.
Magnussen has also been cast as a series regular in the HBO drama "The Money," about wealth and corruption among the super elite.
Actors in talks for the role of Grey also include Luke Bracey, Christian Cooke and François Arnaud, among others.