Mbappe hobbled off in the first half of Friday’s game after a dreadful tackle by Saint-Etienne captain Loic Perrin.
And after tests PSG revealed that their 21-year-old star had “a sprained right ankle with significant ligament damage”.
The news is a major blow for PSG with Mbappe out of next Friday’s French League Cup final and unlikely to be fit in time for next month’s Champions League quarter-final against Atalanta in Lisbon.
He received lengthy treatment but was eventually substituted after the tackle which earned Perrin a red card.
“Everyone is worried. Everyone who saw the foul is worried. Of course I’m worried,” said PSG coach Thomas Tuchel after his team’s 1-0 win.
“It did crack a bit for sure,” Mbappe was heard on camera telling French President Emmanuel Macron who was attending the first competitive game in France since the coronavirus lockdown.
PSG face Lyon in the French League Cup final before resuming their European campaign in a straight knockout tournament at the Estadio da Luz on August 12.
Mbappe’s misfortune revived memories of the injuries that forced Neymar, scorer of Friday’s decisive goal, to miss Champions League last-16 ties in both 2018 and 2019.
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