Pastore, who joined leaders PSG from Palermo for a French record €42 million fee, started off with six league goals as his delicate touch and fine tricks helped him grab the limelight.
He has though suffered a dip in form, scoring only once in a month and losing his joie de vivre.
"Against Nancy [last Sunday], I was not feeling well, physically and mentally," Pastore told French daily Le Parisien on Wednesday, citing fatigue after being called up for international duty with Argentina.
"For evey call-up, it's 13 hours on a plane to get there, and the same amount of time to come back," he said.
PSG fans need not worry, however, as one of the highlights of their season is coming up.
"If last Sunday we had played against Marseille, I don't think I would have been that bad," Pastore explained.
"Generally, I want to give a bit more during these big games."
Big-spending PSG lead surprise second-placed side Montpellier, who travel to Sochaux on Saturday, on goal difference while Marseille, beaten 1-0 at home by Olympiakos Piraeus in Wednesday's Champions League match, are 10th after a bad start.
"We don't know our limits," Montpellier defender Benjamin Stambouli told the club's website.
"We are able to beat anyone but we can also completely screw up any game."
Should that happen, champions Lille will be looking to narrow the gap as they lie third, five points off the pace before hosting Stade Brest on Saturday.
Fifth-placed Olympique Lyon will hope to bounce back from a 0-0 draw against Ajax Amsterdam that left them on the brink of elimination from the Champions League when they travel to struggling AJ Auxerre on Sunday.
The seven-times French champions trail PSG by seven points.
