There is an interesting article up about a new visualization tool in baseball savant around spin axis.
I'm still trying to take it all in, but they do mention Cesar Valdez's "dead fish" pitch that is unlike any other.
Forget what he looks like, just go by his statcast metrics:
38th in sprint speed
36th in Outs above average
28th in OF jump
Now he did come out average catch % added overall which means he's not awful out there, but his metrics tell us it's more than he's an awkward looking chubby guy out there with an inconsistent arm.
In my mind he's a DH that you can play in the corners on occasion but it will be hit or miss whether he will do something to hurt you out there.
You have nothing to lose by not giving Stewart 400 at bats this year.
He does have power and he will likely have a good OBP. We need some of that.
I personally don’t feel he is some awful OFer but I also think I would prefer to see other players out there but he can fill in out there from time to time if needs be.