Bernard Laporte drafted the 22-year-old after Michalak sustained a neck injury during Les Bleus' 35-17 win over Ireland.
And Peyrelongue, originally called up for the Under-21s, was stunned at the sudden step-up.
"Every player dreams of playing with the national team," he said. "I am very happy to be here though this is a surprise as Michalak's withdrawal was decided at the last minute."
Laporte had first considered replacing Michalak with Brian Liebenberg, but in the end decided Peyrelongue was the better choice.
"We had a discussion with Brian Liebenberg but he told us that he had not played at this position for two years," Laporte explained.
"He did not feel comfortable taking up that role. Julien however has all the required qualities to adapt well to the team.
"He is a good defender, a good passer and has progressed a lot in his kicking game.
"He has performed great matches with his club. The only problem is that he is not a goalkicker but this will not unsettle our tactical game."
The goalkicking responsibilities will instead fall to scrum-half Jean-Baptiste Elissalde, Michalak's team-mate at Toulouse.
Peyrelongue's Biarritz colleague Serge Betsen is confident Laporte was right to bring in the youngster.
"Julien is competent and very demanding with himself," the forward said.
"I am lucky to play with him at Biarritz where he is a key element to the team's tactics."