The Italian boss was sacked on Monday having failed to retain the Premier League title he won last season and after losing the FA Cup final to Wigan Athletic.
Mancini took out a full-page advertisement in the Manchester Evening News on Saturday, thanking fans for their support during his time in charge.
The former Inter Milan manager said in the ad: "Manchester City Supporters 3 unforgettable years, you will always be in my heart, ciao, Roberto Mancini."
Ferguson, who will retire following Sunday's game at West Bromwich Albion, rallied behind his former rival and questioned the logic behind sacking a manager who won the Premier League and FA Cup during his three years in charge.
"It was quite amazing. He has won the FA Cup, been in the final, second in the league and won the league and it's not good enough?" he told reporters.
"I was surprised, but actually no, you can't say you are surprised not with some owners today. You can't be surprised, but I still don't think it was right."
Ferguson will be replaced by David Moyes, who will leave Everton at the end of the season after 11 years in charge.
While Ferguson says Moyes will not be distracted by the situation at Manchester City, he will need to adapt to working at a club with a larger global appeal.
"What happens at Man City won't have any bearing on David," Ferguson said.
"He's got the structure here and he's got the experience. He's had to handle things at Everton that are more difficult probably. I don't think that matters.
"I think the enormity of the club will be the most difficult aspect, he will soon realise that anyway.
"The global brand and number of sponsors we have, he has to fit into that. I don't think that's an issue though and the most important thing is the team.
"He's got a good squad of players and he will want to add to that. He will have his own ideas and that's good. He will be fine."