Paris Saint-Germain star Neymar was taken off the field in visible pain on Sunday after falling victim to a heavy tackle from Lyon's Thiago Mendes.
The reigning Ligue 1 champions were far from their best at the weekend as they sought to retake their place at the top of the pile following Lille's victory over Bordeaux earlier on Sunday.
Instead they fell at home to Lyon, who scored the only goal of a 1-0 win through Tinotenda Kadewere midway through the first half to inflict a fourth league defeat of the season on the holders.
That victory allowed the visitors to leapfrog their rivals in the standings, joining Lille at the top one point clear of Thomas Tuchel's men.
But the real cost of Sunday's clash may only become apparent later on as PSG risk losing a key player.
Neymar was scythed down by Brazilian compatriot Mendes deep into injury time as he attempted to engineer another chance to equalise for the hosts at Parc des Princes.
The midfielder crashed into Neymar's ankle, sending him flying to the floor in obvious pain.
Neymar was treated by medics on the Parc des Princes turf but it quickly became apparent that he would play no further part in the game, with his ankle appearing to be most affected by Mendes' reckless challenge.
He was stretchered off the pitch in tears, although PSG were unable to replace him having already made all five allotted substitutes prior to the incident.
Both teams, therefore, finished with 10 men, as Mendes was sent off by referee Benoit Bastien, although the official blew for full time almost immediately afterwards to confirm Lyon's narrow victory.
PSG and Neymar will now face a nervous wait to find out whether the player's ankle sustained serious damage in the challenge, which could cause him to miss a significant period of the 2020-21 season.
The French champions still face three further Ligue 1 games in 2020 before the season enters its winter break, with Wednesday's clash against Lorient at Parc des Princes followed by subsequent matches away to title rivals Lille and at home to Strasbourg prior to the festive period.