The Brazil international completed a 50 million euro move to Paris Saint-Germain last month, but has come under criticism following his performances at the World Cup.
The Brazil international left Chelsea to join the Ligue 1 champion on a five-year deal for a fee reported to be in the region of 50 million euros and end a three-year spell at Stamford Bridge.
Luiz was heavily criticized in the wake of the host nation's humiliating 7-1 World Cup semifinal defeat to Germany and Mourinho says Chelsea does not need the assets the 27-year-old brought to the side last season.
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"Last season he was not regular first choice for us. As a central defender not at all, it was John Terry and Gary Cahill the whole season," Mourinho told reporters. "In midfield, he was a physical guy and gave us important things, especially in the Champions League when [the cup-tied Nemanja] Matic couldn't play. But this season Matic can play in the Champions League so we don't lose that physicality.
"The fact we have Cesc Fabregas moves us in a different direction. It's something that our football needs, and we want to go in that direction. David did important things here, was always a good professional and we will miss him as a good guy. But from a football point of view, we believe our squad is stronger this season than it was."