The City manager is baffled that Barcelona and Real Madrid overlooked the gifted midfielder, signed from Valencia in 2009, who has played a dominant role in the club's 100% start to the season.
Mancini said: "Silva is a top, top player. I don't know why he didn't go to Barca or Real Madrid because he's Spanish but we are lucky because he's here.
"If he had gone to Barca two years ago everyone would say he's one of the best players in the world - and he is one of the best players in the world.
"He's different from Messi and Cristiano Ronaldo because they are strikers and score a lot of goals but I think he's the same as Xavi and Iniesta."
City will make their debut in the Champions League on Wednesday when Napoli come to Manchester, and Mancini will hope to reap the dividends of a rotation policy that is one of the benefits of his large and talented squad.
Mancini admits the players may not be happy to kick their heels on the bench - for example Edin Dzeko did not play a minute of City's 3-0 win over Wigan at the weekend despite scoring four goals in the previous match.
Mancini said: "They may not be happy - that's impossible because every player wants to play always, but I'm the manager and I need to take this decision.
"Now we have a difficult match coming up, we play every three days and these players like Edin, Kolarov, Savic, Nasri have played two games in four days and they need to recover.
"We have a long year and I need all the players to be available as much as possible and sometimes it's better that they rest.
"We have the first game in the Champions League for the club, it will be an exciting night but we need to have a focus and make sure we beat them."
Mancini accepts that Serie A is behind the Premier League in terms of quality but insists City should not think they can stroll to victory against the Italian side, who were from the pot of fourth seeds in last month's Champions League draw but have a pedigree in the competition going back to the days when Diego Maradona ruled the roost in Naples.
He added: "I think at this moment Serie A is behind the Premier League but in the Champions League it's different.
"For Napoli it will be the first time in the European Cup after many years, after Maradona, and for them, like us, the first night will be very exciting.
"They are a very good team and the Champions League is not like the Premier League, we only have six games and you need to win always if you want to go in the second stage.
"It's strange because our group is a very hard group and all the teams can finish top or in fourth position."
City of course, have the Maradona connection too now - Sergio Aguero, who has already hit six goals this season, is the Argentinian legend's son-in-law.
Mancini said: "I don't know if Maradona will support Naples or City - he will have mixed feelings!"