Funnily enough the confusion with the Rugby team is understandable. Man United have the nickname 'The Red Devils'. The original Red Devils were Salford Rugby League Fooltball Club as detailed here. Now, I know Wikipedia is not always spot on but in this case they are right.

As to Man U. themselves - I concur with previous comments. I live about 3 miles away from Old Trafford and used to work within a mile of the ground. The supporters are from everywhere but, seemingly, Manchester! Mancunian support tends to be for the other team, Manchester City, although where I am, in Salford, the Reds probably do seem to have a majority following. It may just be that they are noisier! Difficult to tell but, as a non football fan I can be pretty objective about it.

Enjoy the match anyway and I hope you do not get any of the 'high spirits' as 'fans' tend to call them of blocking roads, intimidating vunerable pedestians and urinating in public places!

Cheers

DeG