The Chelsea boss said the out-of-favour Luiz will partner John Terry in Saturday's Premier League derby against strugglers Crystal Palace, allowing Gary Cahill to rest a quadriceps problem.
"Everything is clear. He's our player and we are happy he's back in a moment when we need him and John in defence," Mourinho told a news conference after training on Friday.
"The market is open in January but I don't think we will be buying or selling...There has been no contact from other clubs or from his people."
Luiz, a gifted but occasionally rash defender, returned from international duty last month with a bruised knee, but his appearances had been patchy before that under Mourinho.
He has been linked with a transfer to Barcelona and to European Champions Bayern Munich.
England left back Ashley Cole, also dropped by Mourinho for a run of games before starting Wednesday's 1-0 Champions League win against Steaua Bucharest alongside Luiz, will make way on Saturday for Cesar Azpilicueta.
"I need the team to change again. I need fresh players. I need Ashley to be fresh for Sunderland."
Chelsea are midway through a bruising run of nine games in December including a League Cup quarter-final at Sunderland on Tuesday and Premier League games at leaders Arsenal and at home to second-placed Liverpool either side of Christmas.
"It's important not to kill players by playing them every minute of every game," Mourinho said.
"I won't risk any players so Cahill for example is not involved...to protect a little pain he's feeling in his quadriceps."
(Editing by Toby Davis)
