I think it's the middle aged guys (Nyquist, Tatar, DDK etc.) that are tuning him out if anyone. Veterans outside of Z just can't hack it anymore. The young guys are the only ones besides Z playing well (Larkin's sophomore slump being the exception, but those happen to players regardless of coaching)
And I don't have a problem with this because Mantha has always taken a good response to harsh criticism. He hasn't been the kind to sulk. Basically this is more "Mo, we know you got more in the tank than you're showing.
Kid that gets all Bs suddenly gets a C, you say "wtf, kid, you are better than that. You're grounded for a day". Dummy kid that gets Ds and that's truly his upper limit? Nothing happens when he gets a D.
People need to stop living and dying over one days benching.