I'm not sure, that Neymar will miss this match even though some sports news report the same thing as you said. i didn't find any news from their official page. but if Neymar is confirmed to be absent, Galtier will automatically include Ekitike as the main starting line-up in this match.
but, from several other news sources that I read. Galtier, will play the MNM Trio with a 4-3-1-2 formation pattern. Messi is back as a playmaker, Mbappe and Neymar are a duet on the front lines. while in midfield, there are Vitinha, Danilo, and Sanches, the rest are defensive line players. but don't get me wrong, this is just a predicted line-up that Galtier will apply. which means, it is uncertain and only a prediction.
Meanwhile, several Montpellier players have been ruled out through injury and two others suspended for red cards. even though this is their home match, it looks like Montpellier will find it difficult to keep up with PSG's game. and this fight, belongs to PSG.