Brittain's injury happened just a few weeks ago, while Molloy is only just returning to fitness after breaking his foot against Hibs last season.
O'Donnell has been out of the first team picture for most of the season, although was given a run of games during part of September and October. With his contract up in the summer, this is the last thing he wants as it's likely to be the end of March before he's back challenging for a first team slot.
Story source: BBC Sport