The Croatian playmaker signed the new deal that will keep him at the club until the summer of 2016, on Sunday.
Manchester United are among the clubs who have been linked with a move for the midfielder.
Redknapp told Sky Sports News: “He has always been happy at the club. He is a fantastic player, he is a great lad.
“He comes in every day and trains hard, works hard. He is low maintenance.
“To have him tied up for six years, the chairman has done a marvellous job in doing that deal with him.”
Redknapp added: “He could play in any team in the world in my opinion and you have got to keep your best players.”