I agree that Davis needs a rest. He's either tired or has lost confidence, or both. Or perhaps it's that strong, experienced central defenders like Millwall's are outmuscling him. But just because there's no one else who can play that role doesn't mean that he can't be rested. I seem to remember that some seasons ago Everton played for several months without a big central striker but still scored goals and won matches. It must be possible to change our formation, perhaps using Adomah centrally and bringing in Grealish and O'Hare. If you haven't got a tall striker on the pitch, stop booting the ball up the middle.
Davis will come again, I'm sure of that, but give him a rest now.