Since it has been shown recently that Mjolnir is sentient, what if "the whisper" was said to it and not Thor? Maybe that's why Thor basically says "what did you say" to Fury. He may not have heard it. Given what is going on in Hickman's Avengers, and Fury obviously know what is going on with the destruction of realities, it could have been something like "only the unworthy can destroy the great destroyer" or something like that. Then Mjolnir would choose for the greater good of, well, basically everything, to not allow Thor to lift it. Considering the watcher's ability to see alternate realities, and knowing that Thor would then take Jarnborn (which apparently can kill Celestials) to use it for the upcoming killing mission on Rabum Alal or whatever he is called. Maybe Jarnborn can do what Mjolnir cannot.
At least that makes more sense to me than a whisper making you "unworthy".......