Louis van Gaal has identified Seamus Coleman of Everton as a January transfer target.
A deal would be worth at least £20 million.
United believe it is more likely Coleman can be signed in the summer but are exploring what would represent the considerable coup of signing the 26-year-old next month.
Van Gaal wants to sign a high-class right-back as he has little faith in Rafael da Silva, the only established player in the position in his squad.