Lucas is poised to join Premier League side Tottenham – where former PSG full-back Serge Aurier calls home – due to a lack of playing time in Paris.
Asked about Lucas' looming exit from the French capital, countryman Neymar admitted that he didn't believe his "good friend" had been treated fairly.
"I'm sad, because he's a good friend, he's a quality player who was rarely used," Neymar told reporters.
"I think it's very unfair. He is my brother in football. I wish him the best. Whatever the destination, I hope he will score a lot and he will return to the Brazilian national team."
Neymar continued: "He's not just a friend, he's a very good guy. Here he could have been used a lot more but I'm not the owner of the team, not the owner of Paris. For me, he should never have left Paris."
Neymar was speaking after 10-man PSG defeated Rennes 3-2 in Tuesday's Coupe de la Ligue semi-final.
There were two incidents that angered Rennes players, firstly when Neymar controlled a high ball on his back and flicked it over the head of Benjamin Bourigeaud in a stunning moment of skill.
In stoppage time, Neymar held out his hand to seemingly help Hamari Traore – who was on the floor – only to pull away as the Rennes defender held out his in appreciation.
"They beat me and I play football. They provoke but I also know how to provoke in my own way, with the ball, with my football. I'm not here to hit, I do not know how to do that. I'm defending myself with the ball," Neymar said.
"People will say that I am a good player and that I provoke a lot, but for me it is normal. I know people will talk about that but they have to put themselves in my shoes. It's no good for defenders to provoke me because I'm going to provoke even more. And I will win with my team."