I don't think it's something that requires seeing him in the flesh, even though I have seen him play quite a few times for Valencia. He is not really a unique footballer, but his playing style is one that would be welcomed at any club.
To me, there are different types of attacking midfielders - the "assist poacher" like Özil, the "goal-getter" like Mkhitaryan, and the "build-up specialist" like Silva. He's the perfect type of player to link up the play from the middle to front, someone who provides a direct burst of skill to counter Touré's brilliant deep-passing skills - something that is weirdly overlooked by many fans of other clubs.
Would I say that Silva is a complete attacking midfielder? Not really; he, like Iniesta and Özil amongst others are quite poor with their shooting abilities. From what I have seen of Silva, he isn't the breed of player to consistently support a defence like a left midfielder would in a 4-4-2 - although Pellegrini's system is different, the midfield is not balanced, something I see as a flaw.
But he provides something that other types of attacking midfielders don't. Özil for instance doesn't provide the same abilities deeper in the pitch that Silva does, but he has such incredible vision in the final third that compensates for his deficiencies. When played with "runners" - Cristiano, Walcott, Müller - he's a joy to watch although he is a peripheral type.
Silva isn't. Just like Iniesta, who would be my ideal comparison player. Silky touch on the ball, can wriggle out of danger, is brilliant linking-up play, but just doesn't have the finishing touch.
Theoretically - where I was coming from - Silva is not an irreplaceable player. There are footballers like him, some more refined than others, who can take charge of the left "wing" and provide a key creative outlet, like Silva. Realistically - which is where the others come from - Manchester City won't be able to sign these players. So he is irreplaceable in that case, I guess.
