The Frenchman was on the bench and subbed into the Ligue 1 game at the hour mark for Hugo Ekitike before netting a late winner. Galtier later addressed questions on why Mbappe wasn't in the starting lineup, saying (via Get France Football News) that the star sustained a knock during the international break.
“Regarding Kylian, he took a knock on his hip against Denmark," said Galtier. "He came back on Wednesday after having 48 hours of rest. He still had a bruise on his hip on Saturday. He had this problem, but he remained available for the team.”
The 23-year-old completed 90 minutes of France's UEFA Nations League defeat to Denmark on Monday but was later diagnosed with a problem in the hip, albeit nothing was serious.
He wasn't immediately ruled out, but it raised doubts over his participation against Nice in Ligue 1, which he eventually started from the bench. However, given the goalscoring talisman he is, Mbappe came on as a substitute in the second half and guided his team to victory with a decisive goal seven minutes from time.
The French international extended his top-flight tally for the season to eight goals from as many games. That has brought him level with teammate Neymar atop the league's goalscoring charts.
His winner also put the Parisians back on the top of the league table with 25 points, moving two clear of Marseille, who had gone ahead following their 3-0 win over Angers.
Mbappe's heroics overshadowed by Messi's sensational free-kick for PSG
Kylian Mbappe may have scored the winner for PSG last night, but it's Lionel Messi's stunning free-kick that's on everyone's lips right now.
The Argentine broke the deadlock for the Parisians in the 29th minute by whipping an unstoppable effort into the top corner from 25 yards out, leaving Nice goalkeeper Kasper Schmeichel rooted to his spot. That was Messi's 60th career direct free-kick for club and country.
The technique, power and accuracy was top notch as Messi demonstrated his set-piece prowess once again. It was his first free-kick in PSG colours, and fans will hope to see many more such goals from him in the future.