Chelsea legend Frank Lampard insists no-one is to blame for Kevin de Bruyne's departure.
De Bruyne proved Manchester City's matchwinner against his old club on Saturday.
Speaking on BT Sport, Lampard revealed some of the details behind De Bruyne's exit, he said: "I think it was a difficult time for the club because they had Hazard, they had Oscar and Willian had just signed so you couldn't play them all.
"And Kevin De Bruyne, his development wasn't where it is now. He was very good at a lot of things in his game and now I think he's great at a lot of things in his game.
"Of course it's easy in retrospect to look at Chelsea and ask them why did you let him go but sometimes players develop at different rates - and Kevin has gone away and made himself a world class player.
On whether Lampard saw it for himself on the training ground, he said: "You don't know, you always wonder about development. He had good pace, he had good passing, he had good dribbling skills but at that point he couldn't get into our team.
"And the one thing he had was a lot of ambition. You could see he was upset that he wasn't playing and I don't mind that in a young player he wanted to go away and prove that he's a player."