City have an impressive squad, and finished second in the Premier League last season.
But their defence let them down on many occasions, and they want a top class partner for Aymeric Laporte.
"I think the unknown is also what level of consistency that Ake will bring. He could be exceptional and make very little mistakes, but Koulibaly is a strong, aggressive player who is good and the ball and when City do the work in reaching out for a player, they typically tick the boxes in terms of the qualities that are needed.
"I think he is probably someone they're looking at from a Champions League point of view to be solid but also to be consistent in Premier League because you're going to need squad rotation, especially with not really having a pre-season."