Just because some posters want the club to buy a RB doesn't mean that Rafael will be sold. If he's not up to the fight then he shouldn't be at the club, I believe he will, he's United through and through. A bit of healthy competition can only be beneficial. Sometimes players get to comfortable when they know they will be playing regardless of form.
You seem to be missing the point here, when Rafael needed cover this season we've had to rely on Smalling, a top club shouldn't have to do that. The last thing we want to do is buy another ' Buttner' type player, or to rely on Smalling/Jones/Valencia which hasn't worked well.
Coleman hasn't been over rated on here though, in fact most have said the 20-25 million asking price is far to much.
Again you mention about 'replacing Rafael' that's not what I believe the vast majority want, they want competition for all places in the squad, Rafael and Evra don't have that at the moment, in fairness there are quite a few players in the team that have no competition.