The former Inter boss spent four years at the Etihad Stadium, leading them to one Premier League and one FA Cup before being sacked at the end of last term following a costly final loss to Wigan.
He also failed to negotiate the Champions League group stage during his spell with the club, but he says he remains proud of his work when he was in England.
"It's a good occasion for me because I have good memories of England," he told a press conference before the Stamford Bridge leg.
"I did a very good job with Manchester City and I'm very happy to be back and in London in the Champions League."
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"It's difficult to replace Drogba. He's a fantastic man and a fantastic player."
The sides drew 1-1 in the first leg in Istanbul and the Italian hopes Gala show less respect to the Premier League leaders as they bid to continue their European campaign.
"We showed a lot of respect for them [Chelsea] in the first 20 minutes [of the first leg], probably because we were nervous. The mentality is important – the attitude that we have when we start this game," Mancini added.
"We produced a miracle in the group because it was a very tough section.
"Now we are here and we know it will be really difficult because Chelsea are one of the best teams in Europe. But in football you never say never. Anything can happen if you believe in yourself.
"We know it will be very tough but we want to do our best. The game will be difficult for both teams, but Chelsea have a small advantage. I don't know if it will be enough to score one goal but we'll see what happens."