PSG, who are six points clear of second-placed Lens with three games remaining this season, lost three out of four home games during a poor run in March and April including a 3-1 defeat by Lorient.
"The fact that we played a fairly successful match at Troyes and that we were able to win against Ajaccio last weekend, that takes the pressure off a little. But the players have been very serious in their work throughout the week."
Messi and Mbappe have both missed league games due to minor injuries, while Neymar has been ruled out for the rest of the season after ankle surgery in March.
"Concerning Messi, Mbappe and Neymar, as soon as I took over it was an objective to be able to associate them together," Galtier said.
"Unfortunately, with the World Cup, injuries and fatigue, we were not able to take advantage of them throughout the season.
"Every time they were on the pitch, PSG were very good. We had a very interesting attacking game. But that's part of a season."
Brazilian defender Marquinhos (29), who has become a first-team regular since joining PSG in 2013 and was appointed captain in 2020, signed a long-term contract extension until 2028 on Friday.
Galtier hailed the announcement, adding, "he is one of the best central defenders in the world. He has been omnipresent for ten years.
"It's a strong signal to everyone. PSG want to remain competitive."