Both Premiership giants are chasing the Dinamo Zagreb midfielder, who admits he wants a move to England in January's winter market.
Durante, who has former Croatia international Milan Rapaic on his books, said: "He is an extraordinary player, the best midfield player in the world that exists.
"People talk forever about Frank Lampard, but you must realize that this is a boy from 85 and already has more than 35 appearances with the Croatian national team. This player can do everything.
"I'm surprised a team like AC Milan, which has opened doors in Croatia, has not thought of this player. Certainly today you'd have to pay £15 million for him."