Mbappe to miss PSG's Champions League tie against Bayern with injury
The France star was hurt during the first half of Wednesday's 3-1 win at Montpellier in Ligue 1 having earlier had two penalties saved.
PSG coach Christophe Galtier had initially suggested Mbappe had suffered a knock to the back of the knee rather than a muscle injury, but tests have since confirmed the true extent of the problem.
"After undergoing tests, Kylian Mbappe suffered an injury to the left thigh at the level of the femoral biceps," said a club statement.
If he misses three weeks as expected, Mbappe can also expect to sit out next midweek's French Cup last-16 tie against Marseille and faces a race against time to return for a potentially pivotal Ligue 1 game away to the same opponents at the end of this month.
However, PSG will hope that Mbappe, who has scored 25 goals in 26 appearances for his club this season, will be fit for the second leg against Bayern in Germany on March 8.
The news is a bitter blow for the French giants, who are desperate to avoid a repeat of what happened last season when they were knocked out of the Champions League in the last 16 by Real Madrid.
The Qatar-owned club have become used to injury crises involving their star players ahead of key Champions League games in recent years.
A fractured metatarsal prevented Neymar from playing a full part in a last-16 defeat to Madrid in 2018 and a similar problem ruled him out of their loss to Manchester United at the same stage a year later.
PSG are hoping to win European club football's most prestigious competition for the first time after over a decade of trying since the Qatari takeover of 2011.
They have been eliminated in the last 16 in four of the last six seasons, although they reached the final in 2020, losing to Bayern in Lisbon, and were beaten semi-finalists a year later.
PSG convinced Mbappe to stay at the club last year and sign a new contract despite interest in him from Real as they hoped he would lead them to glory in Europe.
Mbappe's injury comes just over a month after he starred at the World Cup in Qatar, finishing as the tournament's top scorer with eight goals including a hat-trick in the final, which France lost on penalties to Argentina.
The timing is even more of a blow for PSG after they endured a frustrating end to the January transfer window on Tuesday when they failed to process the paperwork to complete the loan signing of Hakim Ziyech from Chelsea.
The Moroccan winger had already passed a medical and was set to add an extra option in attack for Galtier after the departure of another forward, Pablo Sarabia, to Wolverhampton Wanderers.
