The France international returned to training after being sidelined by acute gastroenteritis last week, and his manager explained that the decision to rest him for Thursday’s match in Philadelphia.
"It was his first day back in boots, his first day running a little, and he did well but he's not fully recovered yet," Alonso told reporters.
"We talked about it after training and he prefers to rest and recover properly for when we go through. And if we make it to the round of 16, he'll be available, but he was still in the recovery process and wasn't perfect for Salzburg."
Mbappe, who was treated in a Miami hospital has already missed Real Madrid’s draw with Al-Hilal and their win over Pachuca.
