As much as I love Mancini, if I think about it rationally, I can't really complain too much about the decision to sack him. This season has been a massive disappointment. It's not that we failed to win anything, it's the manner in which we failed to win anything. We seemed to effectively give up our title in about February. We didn't get a single win in the CL. And we lost the FA Cup final to a team in the relegation zone, seemingly not giving a shit the entire time. And yes, the summer transfer performance wasn't exactly great and he didn't get all the players he hoped for. But it's not as if the team that beat us to the title is a squad of world class players. Yes, they got Van Persie, but they've still got some pretty limited players in their squad, sometimes in pretty important positions. Garcia and Sinclair might not be world class players, but they're not so shit that they couldn't be included in a team that should beat at least 15 of the teams in the league.
Personally, I'd have given him another year to put it right, but I think that'd probably be an emotional decision.