There are pros and cons to going to a 6 man, taxing the bullpen is one of them. You might have more flexibility though, with guys coming back and depth, that is debateable.
Fatigue doesn't go away, you don't push through it. If you've been up for 24 hours and our tired, you don't get less tired by just sticking it out and staying up even longer, you just get more tired. At the end of the day......it's going to get worse if they don't get some rest. I'm not sure if it's them not being more than 120 inning guys or just having a higher work load, I think most pitchers would experience some kind of fatigue once they start pushing past their limits. Tanner has barely ever pitched above 100 innings. Must guys aren't going to go past 100 to 180 innings AND then go deep in the playoffs. With the playoffs you're doubling their load.
I'll be more concerned if he's gassing out around 130-150 innings next year, rather than this year.
But getting more rest should reverse the symptoms