Just got this from the Wikipedia PUP Page, and it sounds like Geno might be eligible after we set the 53, because he wasn't injured in football related activities, he was injured in a locker-room brawl where his jaw was broken by a punch.
Anyway, if we can use this for Geno, it opens a spot to bring back Johnson or Flynn and Geno is eligible to be activated whenever he gets healthy:
https://en.wikipedia.org/wiki/Physically_unable_to_perform
NON-FOOTBALL INJURY
A similar list, known as the "non-football injury" (NFI) list, is functionally equivalent to PUP, but is used for players who are unable to practice as a result of conditions unrelated to football. For example, New England Patriots tackle Marcus Cannon began his rookie season on the NFI list as he recovered from chemotherapy for non-Hodgkin lymphoma. Also, David Quessenberry of the Houston Texans was put on the NFI list in mid-June, 2014 after being diagnosed with non-Hodgkin's lymphoma at the age of 23.
The NFI designation, despite its name, also covers injuries sustained during football played outside the NFL, including college football. Willis McGahee spent his entire rookie season on the Buffalo Bills' NFI list due to the major knee injury he sustained in the 2003 Fiesta Bowl.