The Brazil international signed a four-year deal with the Ligue 1 side, which will see him link up with the squad in January.
"Everything was agreed between Sao Paulo and Manchester United," Ribeiro told UOL. "The contract had been written and just needed to be signed, but then Leonardo called me and asked to talk to Lucas.
"They talked. Leonardo was helped by the fact he is Brazilian. He was very gentle with Lucas.
"The possibility of living in Paris - a much more pleasant city than Manchester - also influenced his decision, but it was PSG's project that impressed Lucas."
Lucas has made 15 appearances for Brazil.