Mancini has come under fire in some quarters for failing to put up a creditable defence of their Premier League title, with rivals Manchester United on course to win the title this season.
Critics have also pointed to City's disappointing performance in the Champions League where they failed to make it out of the group stage.
But Keegan, who managed the club from 2001 to 2005, feels the Italian's achievement in securing City's first league title in 34 years and the FA Cup in 2011 should be acknowledged fully and earn him more time in the Etihad Stadium hotseat.
"In any other sport it would be ridiculous to question someone who has gone into a football club, won the first trophy in 30-odd years, then won the league in the most exciting season ever," said Keegan.
"Europe was a disappointment, but Europe was disappointing for Manchester United and Chelsea. You get that.
"You can make stories around them all you want. Is it right? No. Can it happen? Yes.
"But I don't think there should even be a question of anything happening to Mancini."
Mancini has also been criticised for poor acquisitions in the transfer window, including the signings of Maicon, Javi Garcia, Jack Rodwell and Scott Sinclair, who have failed to make a major impact on the team.
But Keegan believes it may be too early to judge whether those players are indeed flops, while Yaya Toure's decision to sign a new contract is crucial in Keegan's eyes.
"The media can say that it hasn't worked because he bought five players and they didn't win the title," he added.
"That looks like they've taken a step backwards but some of those players might be a lot better next season.
"Players get judged purely on the football field but they've got to get their families over here, get settled and, in certain cases, learn the language.
"Mancini will want to make more signings this summer to keep everyone on their toes.
"But (Yaya) Toure has just signed for another four years and he's a key player."