It is likely that Villa will be willing to sell come the opening of the January transfer window. Cahill moved to United in search of first team football and he has been getting that thus far at the Blades and is likely to be in favour of a permanent deal to the club. The 21-year old is ironically a fan of arch rivals Sheffield Wednesday.
I think this is a good move for both United and Cahill because he clearly has a big future ahead of him and I think it's vital for him to be getting first team football. At Villa he was only going to be used as a substitute at most and I think the move to the Championship will only make him better to come back to the Premier League in the future.