Bobby Valentine has been hoping for a third catcher on the expanded September roster, but the Red Sox didn't have a third catcher on the 40-man, and the PawSox are currently using Dan Butler, the most likely next-in-line, in the International League playoffs. Enter Guillermo Quiroz, who the Red Sox acquired from the Mariners, the team they are currently facing. Quiroz came to the Red Sox in exchange for cash only, and is now on the active roster.
Having a third catcher is generally a luxury in September, but with the Red Sox trying to get Ryan Lavarnway consistent playing time, and without limiting starting backstop Jarrod Saltalamacchia's workload, the two sometimes play together. This means that there is no backup catcher, should something go awry, and while that might be something you just deal with when the time comes for most of the year, the expanded rosters of September allow for backup plans to be present.
Quiroz has spent the entire season at Triple-A Tacoma, and hasn't been in the majors since 2010. He has some solid numbers for a backstop in the minors in his career, hitting .260/.323/.424 at nine seasons at Triple-A, but in the majors, he's been horrendous, posting a 44 OPS+, and compiling all of 101 games played over the course of seven years. Because of this, the 30-year-old Quiroz will likely be placed in a glass case, only to be broken open in the case of emergency.
Quiroz has more experience catching (granted, mostly in the minors) than either Salty or Lavarnway, and it isn't his bat keeping him in the game, so maybe having him around for a month just to talk shop will be good for the two future Sox backstops.