The Brazil international has been in sensational form at the start of the 2022-2023 campaign, scoring seven goals in five appearances prior to Sunday's (AEST) game at the Stade de la Beaujoire.
However, he watched on from the substitutes' bench as Kylian Mbappe put the Ligue 1 champions 2-0 up either side of a red card for Nantes wing-back Fabio, with Neymar introduced in place of Mbappe in the second half.
Nuno Mendes added a third after Neymar's shot had cannoned back off the post as PSG racked up 24 goals after their first six games.
That equalled the record for goals scored by a team in Europe's big five leagues at this stage of a season in the 21st century (Manchester City in 2019-20 and Bayern Munich in 2020-21).
Galtier rejected suggestions Neymar had been dropped, instead stating it was merely a consequence of normal squad rotation.
"You say that all this is new, but it is an obligation in relation to the busy schedule for everyone," he told Canal +.
"We play a lot, every three days then every four days. Everyone understands that they will not be able to play all the 95-minute matches.
"I spoke about it twice. The first time with each player, the second with the whole group, to tell them that it was okay to be like that and that you had to have the right attitude. And also value the participation of your team-mates."
Fabio's red card was his fourth since his Ligue 1 debut in September 2018 – only his team-mate Andrei Girotto (five) has a worse record in Ligue 1 over this period.
It came following an ugly lunge on PSG midfielder Vitinha, which saw the former Porto man immediately require treatment before being replaced by Renato Sanches.
PSG starts its UEFA Champions League campaign at home to Juventus on Wednesday and Galtier is unsure whether the 22-year-old will be fit enough to play.
"He took a big hit on the kneecap, there is no twisting, no ligament risk, we will see if he can play on Tuesday," the PSG boss said.
"It's coming fast, it's close. We will make sure to restore him well and see how he feels. Otherwise, we'll do it with someone else."