The new signing pulled up in pain just 11 minutes into the 1-1 draw at the Lamex Stadium on Saturday.
Posh, who have already lost left-back Tommy Rowe to a fractured collar-bone this summer, will send the former Wolves player for a scan on Monday but boss Darren Ferguson is almost certain Mendez-Laing will miss the early weeks of the new Championship campaign.
"Nat's injury is a big worry," Ferguson told the media post-match. "We'll get him a scan on Monday to find out the extent of the injury but everyone here could see he pulled his hamstring.
"It's very unlikely he'll make the start of the season. You're looking at a couple of weeks out as the best case scenario and if he has torn it badly it will be a lot longer.
"I never like to see muscle injuries and it is something he has had problems with in the past but his medical and scan before we signed him were all clear."
