The Brazilian was initially suspended for nine months by the French league (LFP) after he was found to have deliberately barged referee Alexandre Castro, with the ban extended for the whole of the upcoming season after an appeal.
Without access to the dugout or the referee's dressing room on a matchday, Leonardo felt the restrictions would prevent him doing his job to the full and so reluctantly tendered his resignation to the Ligue 1 champions.
"It was impossible to continue my work in Paris because this suspension would not let me give it 100 per cent," Leonardo told Le Parisien.
"What they did to me was very unjust. Everything became too difficult after this suspension.
"I was the main man responsible for the project at PSG, I was very involved with the players and coaches. It's the most exciting project in Europe today."
Nowhere is that more the case than in PSG's attack, with Edinson Cavani - the leading scorer in Italy's Serie A last season with 29 goals - recruited from Napoli to partner Zlatan Ibrahimovic and with a strong supporting cast around them.
"To have the top scorer in Serie A, Cavani, and the top scorer in Ligue 1, Ibrahimovic, in the same team is incredible," said Leonardo.
"Especially when you can also count on players like Lucas (Moura), (Javier) Pastore, (Jeremy) Menez and (Ezequiel) Lavezzi. PSG have the best attack in the world with those six players.
"It was my idea to bring Cavani and Ibrahimovic together and I insisted it happened.
"Ibra is big and strong but is capable of dropping into midfield and creating (chances). He will play behind and around Cavani, who will be at the front. If you think back to last season, Ibra often dropped deep to use his creativity and we lacked a goalscorer in the middle to get on the end of crosses."
Another "project" backed by an influx of foreign money will hope to prevent PSG retaining their title, with newly-promoted Monaco using owner Dmitri Rybolovlev's vast fortune to attract the likes of Radamel Falcao, James Rodriguez and Joao Moutinho.
"It will not be easy for Monaco but with the players they have recruited, anything is possible," said Leonardo.
"When you look at their friendlies, they are already at a very high level. Monaco will be a serious rival along with Lyon and Marseille who, unlike Monaco, have the advantage of a settled side."