PSG were knocked off top spot following that loss on Sunday, whereas Lorient managed to lift themselves out of the relegation zone with a 3-0 win over Nimes.
As if dropping down two places in the league table was not bad enough, Thomas Tuchel's evening went from bad to downright devastating after Neymar was stretched off the field following a poor challenge from Lyon's Thiago Mendes, who was rightly given his marching orders.
The Brazilian's injury is not thought to be as bad as initially feared, but the die had been cast long before that in the game as PSG fell to a 1-0 defeat on their own turf, with Tino Kadewere chalking up the game's only goal in the first half as Lyon leapfrogged PSG in the table.
The champions also witnessed Lille overcome Bordeaux at the weekend, with Les Dogues now leading the way with 29 points compared to PSG's 28, and Lyon are sandwiched in between the two sides following their crucial win in the French capital.
Tuchel will be demanding that his players rectify the weekend's mistakes straight away against an out-of-sorts Lorient outfit, with the defeat to Lyon ending PSG's five-game unbeaten run on home soil since they lost to Marseille in September's Classique.
Last season's Champions League finalists will also be involved in the pick of the last-16 ties as they were drawn against Barcelona on Monday, but Tuchel will be giving no consideration to PSG's European fortunes just yet as the Ligue 1 champions look to put things right domestically.
In contrast, Lorient will go into their clash with the champions brimming with confidence following a 3-0 win over Nimes - a result which lifted them above the dotted line at their opponents' expense.
Quentin Boisgard, Pierre-Yves Hamel and Yoane Wissa got the goals which eased some of the pressure on Christophe Pelissier, who finally witnessed his side triumph after a six-game winless run in Ligue 1.
As mentioned, Lorient climbed out of the relegation zone with that win, although Nimes are only behind Pelissier's men on goal difference before they play host to an out-of-sorts Nice on Wednesday, and Les Crocodiles will certainly feel more confident about their chances of winning this week than Lorient will.
Lorient lasted tasted success against PSG in August 2011, with the champions winning 12 of the next 13 against this week's visitors to the Parc des Princes.
Paris Saint-Germain Ligue 1 form: WWLDWL
Paris Saint-Germain form (all competitions): WDWWWL
Lorient Ligue 1 form: DLLLLW
Neymar's initial assessment of his ankle injury on Monday showed optimistic results, but further examinations will be required before it can be determined how long the attacker will be sidelined for.
The Brazil international will definitely miss out here though, as will Abdou Diallo, who is set to remain in the treatment room until the new year with a hamstring problem he sustained in the same game.
Paris Saint-Germain possible starting lineup:
Navas; Florenzi, Kimpembe, Kehrer, Kurzawa; Gueye, Verratti, Rafinha; Di Maria, Kean, Mbappe
Lorient possible starting lineup:
Nardi; Hergault, Gravillon, Morel, Le Goff; Monconduit, Lemoine, Le Fee; Lauriente, Hamel, Wissa
We say: Paris Saint-Germain 4-0 Lorient
In a week where former PSG manager Gerard Houllier sadly passed away at the age of 73, the champions will no doubt want to go out there and produce a performance that the late Frenchman would have been proud of. Lorient will be buoyed after ending their winless run in impressive fashion against Nimes, but the visitors will be no match for Tuchel's men, who should record a straightforward victory over the strugglers.