Mourinho's 3 seasons in Serie-A as AS Roma coach, it can be said that AS Roma's performance is stable under Mourinho's direction, but only stable and does not show any improvement. Well, maybe AS Roma feels that they have given Mourinho enough opportunities and prefer to replace him. Actually there was an improvement when De Rossi was appointed to replace Mourinho, but it is quite unfortunate that in the new season De Rossi declined.
Either AS Roma is not very patient or what, because they made a decision at the beginning of the season when usually a team has not found their best form. Now AS Roma must start again from 0 with their new coach.
I agree with you, Mourinho performance was stable during his reign as the head coach of as Roma. But Roma are within their rights to change a coach if they feel the coach is not making any significant progress. But at the long run, replacing Mourinho has not been the best decision for As Roma. In as much they are looking for a coach to bring a long term solutions to their struggles, it is imperative for them to also invest in boosting their squad with quality players. Roma’s decision to find a new coach in the middle of the season may affect their performance positively or negatively depends on the technical ability of the new coach and his abilty to adapt to the conditions at the club.