"He will undergo surgery on his ligament next Wednesday. He will be back in October," Puel told a news conference.
"This proves that we must remain careful with young players. We did everything to protect him but this kind of incident can happen."
The 16-year-old, who turned professional in January, scored three goals in 15 league games with Nice this season.
Nice are fifth in the standings, two points from a Champions League preliminary round spot with six games left.