Manchin will most likely lose. A senator with approval ratings barely above water in one of the most solidly Republican states in the country is not 'Likley D'
Source? Harper had him up by big margins in the poll for Governor, despite other Democrats trailing prospective Republicans.
Also I'd dispute that WV is "one of the most solidly Republican states in the country". Yes it is trending R hard and voted against Obama strongly, but there's still enough elasticity there IMO to help Manchin in 2018. Of course it's a wait and see matter, but still....