Duff was a �17m signing from Blackburn in July 2003

The 27-year-old Republic of Ireland international is thought to be close to a move to Tottenham.

Mourinho said: "In my squad there are always a few I want to leave and a lot of players I'm desperate to keep.

"And there are always a couple of players I'd like to keep but I have no courage to keep when I can't promise they'll be playing 75% of the matches."

News conference: Chelsea boss Jose Mourinho

Mourinho added: "I like Damien as a player and as a person. I listened to him for a couple of hours and I could understand what he has in his mind.

Ashley Cole is a good player but we've been connected with every left-back in Europe Jose Mourinho

"After that it becomes business. He's such a good player we will do good business with another club."

Mourinho has already revealed he will make at least one more signing this summer.

That player is likely to be a replacement for left-back Asier del Horno, who is close to a return to Spain, but Mourinho would not say if Arsenal left-back Ashley Cole was a potential target.

"Ashley is an Arsenal player so I'm not going to say anything. We have Wayne Bridge back from Fulham and he is a player that we trust," he said.

"We will see what happens with Asier del Horno and if we sell him there will be space to buy a new player.