"He's at PSG now, but sports-wise, this decision he's taken is a step backwards", the 2002 World Player of the Year said in an interview with former worldwide Zico's youtube channel.
However, Ronaldo, who spent a year at Barcelona himself, has offered a damning assessment of his compatriot's move.
Neymar, 25, completed a world-record £198million move from Barcelona to the French capital last summer and has scored 20 goals in his opening 22 appearances for PSG.
"Sportingly, the decision is a step back, but there are challenges that everyone seeks", the 2002 FIFA World Cup victor said, as quoted by FourFourTwo.
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Neymar's move was sparked in part because of his man-of-the-match display in Barca's stunning 6-1 win over PSG in the second leg of their Champions League round-of-16 tie, which the French began with a 4-0 aggregate lead.
This weekend might provide a sterner test for PSG as they take on Nantes, managed by former Leicester boss Claudio Ranieri, in Ligue 1.
"I played in Barcelona at the time and went to Inter when the Italian championship was much more competitive".
PSG qualified for the semi finals by defeating Amiens 2-0 while Rennes defeated Toulouse 4-2.