I believe Mauer will be about as good in 2018 as in 2017. I think the concussion factors – perhaps not always measurable by medical experts - can linger for years. Joe clearly proved he can still be an effective hitter – and an excellent 1B.
Don't discount the latter: Mauer is an excellent 1B - he scoops up a lot of hurried throws from the other infielders,
The defensive factor at 1B cannot be ignored. It is a more challenging position to play WELL than some other positions. As a 1B, you touch the ball, usually, more often than any other fielder except the pitcher and catcher.
Kennys Vargas has hit well this spring, but I really hope he gets traded – he's just not a very good FIELDING 1B. And PLEASE, please, please do not put Miguel Sano at 1B. If he cannot handle 3B, he should be a DH. He is not a very good defensive 1B, in my opinion.
That leaves Mauer and Morrison at the top of the 1B heap. They will be good defensively, and, I believe, offensively.
When the Twins consider whether or not to bring Mauer back in 2019, I hope they consider his stellar defense in addition to his bat.