I think we're deluding ourselves if we think Moyes can attract any big names in January
Is it reasonable to assume this is such a big factor? Has a player ever argued the name of a manager being an obstacle to a transfer unless there was some personal issue among both?
It may work as a "positive", arguably (i.e., wanting to play under Mourinho or SAF) but don't see it being an obstacle. Premier League and Manchester United have enough pulling power themselves.
You could say United's position at the table might make some players thirsty of European football hesitate, but those would hesitate regardless of the name at the helm.
In the end I think wages still are the biggest pulling power available, the likes of PSG, Monaco, or Eastern European rich clubs have shown us that. After that I'd think the biggest factor is how highly the player perceives his chances of becoming a starter. Stuff like the name of the manager probably comes at the bottom of their concerns.