Eyebrows were raised when Neymar left Lionel Messi and Barca in a world-record deal after French powerhouses PSG activated the Brazil international's €222 million buyout clause in August.
Neymar has starred for PSG, scoring 20 goals in all competitions – 11 of those strikes in Ligue 1 – but in an interview with ex-Brazil international Zico via YouTube, former Barca striker Ronaldo said: "Sportingly, the decision is a step back.
"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."
Neymar was on target on Wednesday as PSG defeated 10-man Amiens 2-0 to reach the Coupe de la Ligue semi-finals.
PSG are back in action against Claudio Ranieri's Nantes in Ligue 1 on Sunday.