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.
"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."
