As others have mentioned, this isn't so much in defense of Fredi as it is a thought about what hasn't been mentioned.
If you're assuming that Vizcaino IS the only reliever potentially available in that situation and there's the very real chance that you may need to get more than one inning from whomever you run out there you have little choice other than using a SP. Given the fact that it WAS Wisler's throw day, he was going to be working in some form no matter what. Even if there wasn't an off day today, he'd have been the obvious choice IMO. You'd have had plenty of options in Gwinnett to call up to start if there had been a game today and skip Matt this time through - PARTICULARLY since Folty was scheduled today. Depending on how things went in a Folty start with the big club, you'd be able to juggle the rotation a little over the next few days to slot Wisler into Perez' slot next time through and move Williams to the pen.
I'm not saying that absolves Fredi of bad pen management earlier in the game, but with at FIVE guys available in Gwinnett (and capable of slotting in at some point in Folty, Gant, Jenkins, Blair, and Kelly - a luxury we haven't had in quite some time), using a starter like that really doesn't scream "fire him NOW" to me.