There used to be, but it's been a long time. Right now you just have to be in the organization by 9/1. Technically you also have to be on the 40 by then but any injured player can be replaced by someone who wasn't.
So for instance, Jose Cuas, who became a Phillies on 9/7, is not playoff-eligible. But Mick Abel could be, if they used him to replace (on-paper) Ortiz.