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 £40 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 services of David Luiz to be successful this 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."