The 21-year-old all but ended months of speculation over his future earlier this week by revealing he would leave French club Lille for Chelsea.
"I think he is a great player," the Arsenal centre-back said.
"Technically he is one of the best in our team. He will do well for Chelsea and I think he will be nice to watch for the spectators when they come to the stadium.
"I don't know [if I am surprised he chose Chelsea ahead of City] because I was not there behind the scenes.
"He made his mind up and I think he will do well, I am convinced he will.
"I am happy he came to England, I think it is good for the national team as well."