Paris St Germain manager Thomas Tuchel said he can understand fans' frustration at Neymar as the Ligue 1 side started their title defence without the Brazilian.
But Neymar, said to be unsettled in France and who has been linked with Barcelona and Real Madrid, was also the focus of attention with a number of fans holding a banner telling him to "get lost" and chanting about the 27-year-old.
Tuchel, quoted in L'Equipe, said: "It's tricky. I don't know what to say. I saw it on my colleagues' smartphones because I did not hear them (the chants) during the match.
"Can I understand it? Yes and no. He's still our player, my player, and I protect all my players.
"He's in my dressing room. I can understand that not everyone has liked what he has said and done."
Cavani's penalty, following a VAR review on 25 minutes, gave PSG an interval lead before second-half goals from Mbappe and Di Maria put the result beyond doubt.
Asked if Neymar's creativity was missing, Tuchel said: "It's always missing because he can find solutions that only he can find. He can open space with movement, an injection of pace, one on ones..."
Cavani put the champions up with a penalty after Nimes defender Pablo Martinez handled in the box with the spot kick awarded by VAR, with Mbappe and Di Maria both finding the net early in the second half.
Mbappe, twice, and Cavani both squandered further opportunities as PSG pressed for a fourth goal late on, while Nimes went close to a consolation in the closing stages through Antonin Bobichon and Renaud Ripart.