He said: "Don't kid yourself, this boy has so much quality it's frightening. It staggers belief to look at where we are now to where we were a few years ago.
"I spoke to both Frank Lampard and John Terry about Diarra and they said he is one of the best midfielders in the country and will probably be worth about £25million in a couple of years' time. He's not some cast off that two top four clubs didn't want.
"Chelsea offered him a huge contract to stay and Arsene Wenger didn't want to sell him.
"But fortunately the boy was adamant he wanted to leave because he wanted to play first-team football week in, week out and he knows he will get that here.
"Yet some people criticised him when he was honest enough to say that eventually he would move on and play for a massive club."