The fans were more sorry to see Mata leave than Matic. Mata was always a favourite whereas Matic had a stinker the year before he left.
The Chelsea to Utd transfer route is a relatively new one and there is a chance Willian will go this window. Personally I think Willian would be more successful at Utd than both Mata and Matic. I know he's getting on but he still has reasonably good energy and his passing at close quarters is both accurate and quick. On his day he also has a good free kick.
While there is a chance he could go to Utd I think Chelsea would rather sell to another league and as a Spanish speaker I think he would rather go to Spain.
The closer we get to the transfer deadline, the more I want Willian at United. I think it will happen, but it's going to cost a lot, £55 million + IMO.
I also think he'll be a good transfer for a few years, whilst hopefully the likes of Chong (a talented youngster on our books) will be at a stage where he is ready to compete for proper first team football by then.