Looks like Matt had to drop out of Manchester-by-the-sea due to schedulling conflicts. Via Collider:
Matt Damon’s busy schedule has forced him to drop out of reteaming with Margaret writer/director Kenneth Lonergan on the drama Manchester-by-the-Sea, but a friendly face will be filling in. Casey Affleck revealed to the Boston Globe (via The Playlist) that he’ll be replacing Damon as the lead in Lonergan’s new film, which revolves around a Boston plumber who is forced to look after his 16-year-old nephew after his brother passes away, but is hindered by a secret tragedy in his past. The You Can Count on Me writer/director Lonergan is an incredibly talented filmmaker, so his next effort is anticipated by many (myself included). No word on when production might begin.
Damon is currently leading Ridley Scott’s sci-fi drama The Martian and is poised to reteam with Paul Greengrass on another Bourne film for release in 2016, so fitting Manchester-by-the-Sea into his schedule ultimately proved impossible.