Luiz has been linked with both Barcelona and Bayern Munich during different stages of the close season, with the Catalan giants looking for a central defender.
Mourinho has rubbished talk that Luiz may leave West London before the transfer window closes, declaring himself impressed with the centre-back's early training sessions since joining the Blues on tour in the USA.
He told The Mirror: "David has come back with us fantastically and we have to use his quality.
"He has big self-confidence and self-esteem, especially when he comes up from the back with the ball.
"Can he improve defensively? Of course. But it is my job to try always to improve players."
Luiz was rested as a precaution during yesterday's 2-0 win over AC Milan after suffering a minor hamstring injury, but is expected to feature before Chelsea's pre-season tour finishes against Roma on Saturday.