Mats Hummels (Borussia Dortmund)
A player long linked with a move to Old Trafford is Borussia Dortmund central defender Mats Hummels, but the struggles of the Bundesliga club make a swift departure for the German international increasingly likely. United have been less convincing at the back this season, on account of both inexperience and a succession of unfortunate injuries, and Hummels is the man identified to step in and rectify that particular issue.