I'm surprised no one has mentioned this yet - it's a 'theory' that claims that everyone is promoted to his/her highest level of incompetence. That being the case, your manager may be feeling somewhat 'out-on-a-limb' himself. Finding someone else being badmouthed makes him feel a little better and if someone else is getting all the negative attention, then less negative attention is directed at him.
I'd also have to disagree with folks who say "Just do the best job you can" and wholeheartedly agree with those who advised "Get the heck out of there." You do you job as best you can, your manager, who may well be a flaming moron, becomes more and more threatened. Now he may actually solicit negative testimonials.
It does happen all over the place - pretty much everywhere I've worked in the 50 years I've been around. Like my Daddy always told me: "I never said it was fair; I said it was life."
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