The Brazil international left Chelsea to join the Ligue 1 champions on a five-year deal for a fee reported to be in the region of €50 million and end a three-year spell at Stamford Bridge.
Luiz was heavily criticised in the wake of the host nation's humiliating 7-1 World Cup semi-final defeat to Germany and Mourinho says Chelsea do not need the assets the 27-year-old brought to the side last season.
"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."