Mancini's days were numbered from the minute Cook went out the door. Soriano was in and just like many other clubs, directions change.
Mancini was getting better with us each year, we were making progress, right up until the players became bigger than the manager. Now going by the history books, managers who tend to come across as absolute bullies in clubs do very well. Ferguson had a stranglehold of THAT club and in some aspect still does. If you questioned the manager, you were out the door. And that's how it should be.
2012/13 was a strange season though, the summer transfers being the biggest factor of them all. Players were coming in that had absolutely no effect whatsoever. Granted that FFP was now in play, but that wasn't an excuse for players to put in so many half arsed performances. In previous years, the board was publicly singing the managers praises, then it all went very quiet. He was undermined the entire season, not just by the board, but by the players aswell. And of course the players (we won't mention any names) got what they wanted and he was gone, paying the ultimate slap in the face not just to the manager, but to the fans aswell with a non performance at Wembley against Wigan.
Mancini was a great manager for us, but it's in the past, he has a relatively good legacy with the club. And one poor season from everyone all around isn't going to tarnish our first league title in 44 years.
As for the original question though on him coming back, right now we have a manager. And so far, this season is looking all very familiar... so going by previous mistakes, the right thing to do would be to back the current manager. Pellegrini can also be a good manager, whose players are currently not giving him their best effort for unknown reasons.