Argentina s Lionel Messi. AFP
Messi, 35, was the lynchpin of the Argentina side that won the World Cup in Qatar nine days ago, scoring twice in the final against France which finished in a penalty shoot-out victory.
He then returned to Argentina with his teammates to celebrate their third World Cup triumph.
"Leo had a very big World Cup with his victory," said Galtier on the eve of PSG's meeting with Strasbourg as Ligue 1 kicks off again on Wednesday after the World Cup break.
"He had to return to Argentina for the celebrations and we decided that he was going to be off until January 1.
"He will come and join us either on the second or third (January) to pick up the season with us.
"He will have had between 13 and 14 days of recovery."
Messi looks likely to miss two Ligue 1 matches and the French Cup match at Chateauroux, possibly making his return at home against Angers on January 11.
He is also expected to sit down with PSG president Nasser Al-Khelaifi and other top club officials to pen a one-season extension to his current deal in the French capital which expires in the summer.
PSG's other World Cup players, notably Kylian Mbappe and Neymar "will be available for the match tomorrow (Wednesday)", said Galtier.
Mbappe could even start, having returned to training just three days after the disappointment of losing the World Cup final, in spite of his hat-trick.
“We made sure to manage on a case-by-case basis,” said Galtier. "After talking with him, Kylian wanted to join us fairly quickly."
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