Jose Mourinho says David Luiz has room to improve - but suggested once more that he is not for sale despite interest from Barcelona.
Barcelona reportedly had a bid for the Brazilian turned down yesterday but could come back in with an improved offer, despite Mourinho's insistence that he is not for sale.
The Special One says Luiz is key to his plans for Chelsea because of his style of defending, but says he must improve certain aspects of his game.
"I have known of David Luiz from the very beginning so I know him very, very well," he told the Sun.
"And before I came back to Chelsea, I watched him week after week.
"I see him as a centre-back because he has an important quality for a footballer - he can build from the back.
"He is very comfortable and confident with using the ball from the back - he does not play direct.
"And he can improve, especially defensively. He can be much more effective, much more solid. But the potential he has is amazing, incredible."
Luiz sat out of Chelsea's 3-1 defeat against Real Madrid in Miami last night with a muscle injury, but could return for the Blues' final pre-season fixture against Roma.
Chelsea get their Premier League campaign underway against Hull in under 10 days time.