Mourinho losing once and the fans are going crazy... . In my opinion, Mourinho saved Roma when he came and the entire city knows it so even if you like Mourinho or you don't , he is one of the greatest coach ever who didn't back down from a challenge like coaching As.Roma when the team was in a free fall. Also , they are the champions of Conference League so I suppose they can afford to lose 1 game...
It's very difficult to find a team that has the performance you want. In other leagues, teams that look superior can still lose. Especially in Serie A, even though AS Roma lost to a team that is not a favourite, this is something we usually find in football. Even if we look at the Premier League, something like this can happen every week.