Sergio is a City legend and will go down as one of the best ever players, almost certainly the best striker, to ever play for the club. He is one of the best goalscorers that this league has ever seen.
However, what he has never been is a player who can create things or particularly good at general team play requiring vision. Even at his peak he was lethal at creating space to shoot but only in sharp burst in the most effective areas, when the team had already got the ball into those positions.
Unfortunately, that is not what this team and this system requires. It requires someone who does have the ability to bring other into play and does have vision. With the whole team having that, the goals take care of themselves (see the amount of goals from non-strikers this season).
So, despite Sergio being much more of an impressive player and the fact that he will be remembered as far better player over the course of his career, a 'good' player like Firmino is far more suited and would be far better in the position in this team. Firmino guards possession and brings his teammates into the game.
Sergio never particularly did either of them at his peak but now, given that he has lost pace and, it seems, confidence, he is actually pretty bad at both. That's not his game and should not detract from his legacy but his control and ability to get his head up is so out of place in this team. It is criminal that the ball just bounces off him so much and that he runs into defenders and blind alleys so often, giving up possession.
This is why Pep wanted Sanchez, why an on form Jesus is better in the position and why players that when football history is written will not ever be compared to the great Sergio Aguero would be better than him in the position. We don't need his avalanche of goals anymore. This team will always score and he isn't anywhere near as sharp as he used to be anyway. The team needs someone who is on the same wavelength and plays in the same style as KDB, Silva and Sterling. And Sergio is not that man. What's more, his body language suggests that even he doesn't believe that he is that man.
Still the best goalscrorer that will ever play for the club.
