Manchester United have agreed to sell Angel Di Maria to Paris St Germain for 63million euros.
The Italian had considerable support in the stands and that was evident at the FA Cup final at Wembley on Saturday when, amid rumours over his future, his name was chanted regularly.
Mancini ended the club's 35-year trophy drought by overseeing their 2011 FA Cup success and bettered that by winning a first top-flight title in 44 years last season.
The disappointment among supporters was soon apparent across social media and internet forums.
That the news of Mancini's departure, after three days of intense speculation, broke on the anniversary of last year's dramatic Premier League title success added to the gloom.
Kevin Parker, general secretary of the Manchester City Supporters Club, said: "After the weekend I think we were all expecting it to happen but I think we are all a bit disappointed it happened yesterday of all days - the anniversary of when we won the Premier League.
"All those fantastic memories we have of May 13, 2012 - that date is a special date for City fans and that's been a little bit tarnished. We're definitely all disappointed."
A statement from the club said that Mancini had been relieved of his duties for failing to achieve the season's "stated targets".
Parker said: "It is unbelievable. The club have said the targets set haven't been met, but finishing second in the Premier League and losing finalists in the FA Cup would suggest those targets might have been unachievable.
"Does that mean we had to win the Premier League and win the FA Cup? I'm not sure."
The club's statement also referred to a "need to develop a holistic approach to all aspects of football at the club".
Parker feels this could be a reference to various reports of disunity within the dressing room during Mancini's tenure.
This season alone Mancini has attracted criticism for seemingly unnecessary comments about Joe Hart, Micah Richards and Vincent Kompany while the many incidents involving the now sold Mario Balotelli were a patience-draining distraction.
Parker said: "I think it seems clear - whilst they might not be happy with what's happened on the field - there are other bits and pieces off the field that we as fans don't get to know about.
"This reference to an 'holistic approach', there is probably a message in there somewhere.
"I think they are looking for peace, harmony and happiness in the camp. That would suggest that under Roberto that isn't the case.
"We all hear rumours and hear stories about it not being a particularly happy camp.
"But as fans our interest is a successful football team, success and trophies, and under Roberto that is what we got.
"Of course we are disappointed we have not won a trophy this season but in the eyes of City fans, that is not failure.
"When you have waited as long as we have for trophies, then the three Roberto has brought in three years are certainly keeping us happy."
Mancini's exit means that Manchester United, Manchester City and Chelsea - the current top three in the Premier League - will all have new managers next term. Sixth-placed Everton will also have a new boss.
Parker said: "Whoever the new manager is is going to be will get the full support of the City fans.
"The appointment of the new manager - in no way is he associated with what has happened to Roberto Mancini.
"But it is just disappointing. I would imagine there would be a lot of chanting for Roberto tonight at Reading and this weekend against Norwich.
"Sir Alex Ferguson and David Moyes had the opportunity to say goodbye to the supporters who backed them, we don't get that.
"I think the writing had been on the wall but it would have been quite nice to have the opportunity to say goodbye to Bobby.
"Football isn't as sentimental as that, unfortunately."