PSG came into their first match of the new domestic season with seven of their star players missing after testing positive for coronavirus, including Neymar and Kylian Mbappe.
They started the match with their third-choice goalkeeper Marcin Bulka and it was the 20-year-old who handed Lens the win with an awful blunder 11 minutes after the break.
Under no pressure and trying to play out from the back, Bulka slid a weak pass towards Marci Verratti that was snapped up by Ignatius Ganago, who slammed home what turned out to be the winner from close range.
The strike was the Cameroon international's first goal for his new club after signing from Nice in July and sent the rowdy 3,800 fans at Stade Bollaert-Delelis wild with joy.
"I'm very happy. It feels good to open my account and I hope to score many more," said the 21-year-old Ganago.
"Even without Neymar or Mbappe Paris have a very good team. We were solid, we were compact, we all worked together and it paid off in the end."
Covid-19 has ripped the heart out of coach Thomas Tuchel's side, with Angel Di Maria and Leandro Paredes also testing positive.
PSG confirmed three more cases with L'Equipe identifying them as Argentine striker Mauro Icardi, Brazilian defender Marquinhos and goalkeeper Keylor Navas.
Apart from Mbappe all of them contracted the virus after holidaying in Ibiza after losing the Champions League final to Bayern Munich in Lisbon last month.
Mbappe tested positive on France duty at the start of the week, forcing the world's second most expensive player to sit out Tuesday's World Cup final rematch with Croatia.
Tuchel's thin squad now face a rush of fixtures as they try to catch up with a league awaiting its third round of matches at the weekend.
PSG take on arch-rivals Marseille at the Parc des Princes on Sunday night before another home match with Metz — which was their original opening fixture — on Wednesday and a trip to Nice the following Sunday.
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