Messi opened the scoring in spectacular fashion as PSG drew 1-1 at Benfica on Thursday (AEDT), becoming the first player to score against 40 different teams in UEFA Champions League history.
He has also been involved in seven goals in his past five Ligue 1 matches (two goals, five assists), but a medical update issued by the club revealed Messi would miss Sunday's (AEDT) match despite undergoing "reassuring examinations" on a calf injury.
"There will be changes," Galtier said at Saturday's (AEDT) pre-match news conference.
"Leo Messi will not be fit for Reims but will resume training on Sunday morning. Kylian Mbappe has angina but will be travelling to Reims."
Meanwhile, Galtier praised Gianluigi Donnarumma for his response to criticism of his early season performances, after the Italy goalkeeper made six saves at Benfica.
"He works well, nobody is airtight against remarks - sometimes it becomes criticism and nobody is insensitive," Galtier said.
"What I could notice in this period is that he remained focused on his work, by trying to eliminate his little flaws and keep his strengths.
"I saw nothing different in his preparation for the Benfica match. Obviously, he and the whole team, we were all satisfied with his performance in Lisbon.
"I always pointed out his few little mistakes, whether it was a lack of concentration or other issues. He was told after Benfica that he must continue to always be efficient."
Should PSG win at Reims, it will have taken 28 points from its first 10 matches of the Ligue 1 campaign.
In the 21st Century, only two teams have reached at least 28 points at that stage of a Ligue 1 season, with both going on to finish as champion – Lyon 28 in 2006-2007, and PSG 30 in 2018-2019.