"As announced after the final of the Coupe de France against Saint-Etienne, Kylian Mbappe underwent further examinations to on his injured right ankle," the club said in a statement on Monday.
"The results of today's scan confirmed an ankle sprain with damage to the external ligament. The player is expected to be sidelined for approximately three weeks."
Mbappe suffered a sprained ankle in the French Cup final against Saint-Etienne on Friday night at the Stade de France, a game PSG won 1-0 thanks to a 14th-minute Neymar goal.
He was injured by an ugly tackle from Loic Perrin before the half-hour mark, but scans and X-rays Mbappe had on Friday night and Saturday morning revealed that no ligaments were damaged.
Mbappe, though, had more scans on Tuesday morning to assess the injury further, and it was determined he could be set to miss the Atalanta match as well as the League Cup final against Lyon on Friday.