From what I can tell, Guillen's trying to give him as much rest as possible and probably trying to limit the chances of aggravating his bad back. This is not the first time that Guillen's tried to push his setup guys to their limits before calling in Hermanson. The leader in my dynasty-like points league owns him (and also Marte at some points) while I owned Politte (until recently) whom I swapped for Jenks off WW, so I've been paying attention to their bullpen situation. I was watching Sportsline's Gamecenter during those final 2 frames yesterday and did notice Hermanson starting to warm up in the bullpen. Couldn't tell whether he was held out because his bad back stopped him or whether Guillen was just being extra cautious as has been the case for maybe a month now.
Me? I'm hoping they give the job to Jenks next year.
Of course, if Jenks stays good next year, he'll probably inherit the job the year after when Hermanson's contract is up. In my dynasty-like points league, holds are worth just as much as saves and run prevention is important, so a top setup guy can be just as valuable as a middling closer.