Pastore hurt as PSG progress in French Cup
Brazilian Nene and Kevin Gameiro scored twice apiece for Carlo Ancelotti's side but Pastore limped off with an apparent thigh strain after 25 minutes.
The Argentine, who joined from Palermo for a French record fee of 42 million euros in August, now faces a spell on the sidelines.
PSG struggled to create clear-cut chances early on against fifth-tier Sable sur Sarthe.
Nene finally broke the deadlock in the 36th minute when he converted a penalty after being brought down in the box.
France striker Gameiro, who had fluffed several opportunities, made it 2-0 in the 64th minute after latching on to a Jeremy Menez cross.
He added his second goal after collecting a defence-splitting pass from Nene in the 73rd minute and the Brazilian wrapped up the scoring in injury time.
"I was not happy with the first half," Ancelotti told Eurosport. "After the break it was better because we played well on the counter attack."