Today Adobe released an update to the ACR plug-in and DNG Converter to support the new Nikon D850 and several Nikon lenses. The equivalent Lightroom update is coming soon (Adobe keeps making it more difficult to download the standalone version, see this download info and also this page for more info). In the meantime, you can use the free DNG Converter on Nikon D850 RAW images prior to Lightroom import:
Download links for the Adobe DNG Converter: Mac OS | Windows OS.
New camera and lens support added in Camera Raw 9.12.1 (direct download link):
D850 pre-orders: B&H | Adorama | Amazon | BuyDig | BestBuy | WEX | Jessops
Facebook: Nikon D850 Page | Nikon D850 Group
Additional coverage: Nikon D850 directory