I would be surprised if we get an answer to that question this year.
He is definitely one of the players, along with DJ Stewart, I most wanted a full season of data on at the MLB level this year and unfortunately we aren't going to get it.
Well, at the moment I don’t think we have anyone who is better, and damn few who are as good. I am happy giving him playing time until someone better comes along, and the better he gets, the better the new guy will have to get.
If Santander plays left and Stewart plays right Mountcastle will never play the outfield. Mountcastle's arm is not good enough to play right and he is not good enough to move Santander out of left.
So Mountcastle better hope that Stewart is starting the season in left field.
As for Mountcastle and Stewart, Mountcastle is going to be playing just about everyday between LF, DH and 1B once the Orioles get the extra year of control (I believe somewhere around 7-10 games). Stewart will get his ABs in LF, Rf and DH as well, but I don't see him as a priority play.
If Santander will be a go in RF, the outfield will typically be Mountcastle, Hays and Santander with Stewart being the 4th OF with Santander moving to CF to give Hays a break against maybe a tough righty.