I was leaning toward being replaced by the change in system. He was told that he wouldn't be guaranteed minutes if he were to stay here. Sergio Aguero, 100 % fit yet still not in the team? why? Because he doesn't suit the way we now play. Our striker is now asked to press relentlessly, drop deep to link play and draw out defences(Jesus not offside very much anymore), interchange and drift wide to overload the wing whilst our midfielders get into the box. These are things you'd struggle to get Serge doing, even in his prime. Now with age and injuries taking their toll, he has become surplus to requirements.
We're looking for someone with a completely different skillset than Aguero possesses. For example Danny Ings would actually suit our current system more than Aguero does, he's not even half the "striker" Serge is but would be a square peg to fit the square hole in our team.