Hairston, a third-year player out of Clemson, missed the team's spring practices as he recuperated from off-season surgery. That rehab is the likely cause for the team's decision to put him on Active/PUP, which will allow Hairston to return to the practice field as soon as he's medically cleared to do so. What isn't clear is how long he'll be sidelined; clearly, the longer he's out, the better the chances are that Erik Pears will open the season as the starting right tackle.
For the moment, Buffalo won't need to create any roster space to accommodate Hairston's inevitable return to the active roster, as they're currently sitting at 88 players out of a maximum of 90 following today's release of Mark Anderson. One of those 88 players is also Jairus Byrd, who may or may not show up for the start of training camp.
Pears returned to the practice field in a limited capacity during the team's final mini-camp in June, and it's expected that he'll be fully healthy for the start of training camp on Sunday.