Neymar was made the world's most expensive player when pushing through a shock £198m transfer in August, more than doubling the previous record fee for a transfer.
The 25-year-old has been in inspired form for the Parisians since making the move, scoring 20 goals in all competitions so far, but Ronaldo has questioned his compatriot's decision to leave Camp Nou in the first place.
"Sportingly, the decision is a step back," he told Zico in a YouTube interview. "But there are challenges that everyone seeks. I played in Barcelona at the time and went to Inter when the Italian championship was much more competitive."
Eleven of Neymar's goal this term have come in Ligue 1, which PSG lead by nine points.