United is more of a cultural phenomenon now than a real juggernaut of the game (they could become a juggernaut again but not with these owners). A contributing factor as to why united may not have lost as much popularity though would be that their darby rivals in city are kind of seen as cheaters in a way. That statement is gonna get some hate, but i mean they are legitimately accused of over 100 breaches of financial fair play spanning back many seasons. City is a fantastic team, but its hard to be very romantic about them in the way people are about united.
Really the fans will probably continue to revolve around the traditional big 6 for the foreseeable future as even when they are performing worse like chelsea and man united, they still bring in tons of big name players that get everyone talking. A lot of the big pundits are also former players of those teams which helps as well.
Imo yes it will put a bit of a stain on his time with the club, but it wont change the fact that hes a great manager. Hes proven himself time and time again to be one of the best managers in the world, and done it with multiple teams. Hes also been one of the most influential figures in modern football. Seems like tons of teams look to his lineups and tactics for ideas of what to do and how to move their team into the future.
So yeah man city being found guilty of record breaking amounts of cheating financially will probably make us think less of the team as a wholes success, and that includes his, but it will not remove the great work hes done as a career.