The elephant in the SOX room isn't Masa's play in LF. It is starting pitching. Imho, if Bloom fails to pick-up a couple of decent starters for next year, then the SOX will be breathing fumes in the AL East again.
I believe Casas has the work habit to make himself into, at least, an average defensive 1st baseman, which combined with his offense would be quite acceptable.
What you say is true regarding today's game, but for the whole season the Rays have been better in all four things, which is reflected in their W/L record compared to the SOX.