Personally, if I'm put to a decision to play a good player not rated at a position or a player who has played a few innings at a position but has poor ratings, I almost always will play the non-rated player. Even downgraded a bit, that player usually will be a better fielder.

This year, I have played Adrian Beltre in left field and he has done a very good job. I have also, when forced by an injury plague at SS, played Vinny Castilla at short. He had a few more errors than a rated SS, but other than that he did a decent job - and has a very good bat for a SS. :o)

In your situation at 1B, I would just use the best hitter you can and not worry too much about defense.