It has been well documented that the Pittsburgh Pirates do worse late in the season, (particularly September) than the season as a whole. This is with a handful of notable exceptions like 2006.
It's a bit dangerous to extrapolate the rest of the season into the last month because the mix of opponents may be different. (For instance, the Pirates went 10-8 in interleague play in June, but haven't done nearly that well against National League teams.)
So I performed a more circumscribed exercise by asking what the Pirates might have done against their September opponents, based on what they had actually done earlier in the year against the same opponents.
The Pirates played five games against the Milwaukee Brewers in September, after having gone 4-6 against them earlier in the season. A simple extrapolation suggests that they should have won two games out of five against this team in the past month, instead of zero.
Pittsburgh had dominated the Chicago Cubs 6-3 before September. That suggests that the Bucs could have gone 5-2 against this team in the past month, instead of 2-5.
The Pirates played six games against the cellar-dwelling Astros this past month, after having gone 9-2 against them earlier. Let's be generous and say that the Houston might have won two of those games, which would suggest four Pittsburgh wins against this team instead of three.
Pittsburgh had lost a series at PNC Park to the New York Mets in May, so I would assume a sweep in Citi Field (even thought the Bucs won one).
The Pirates had gone 6-6 in the season series against Cincinnati up to September, which (together with several close games) suggests that they should have split the last six games 3-3, instead of going 1-5.
Two more wins against the Brewers, three more against the Cubs, two more versus the Reds, one more against the Astros, minus the game they "should" have lost against the Mets. This hypothetical situation would represent seven more September victories for the Pirates and a record of 86-75 going into the last game, almost enough to contend.