Mauer ranks as the second best defensive 1B in the AL according to the new @sabr defensive index totals pic.twitter.com/9dbmwenyIv— Cody Christie ⚾️ (@NoDakTwinsFan) July 15, 2016
There are aspects of this list that are sensible, as aspects that are counterintuitive. For example, looking at the bottom of the list: Hanley Ramirez of the Red Sox is awful. Eric Hosmer ... while the Gold Gloves are not the be-all and end-all of defensive ratings, he's won the last three (although the defensive metrics available on Baseball Reference agree that he's been abysmal this year.)
Mauer near the top of defensive first basemen? Frankly, I haven't seen that coming. The aforementioned metrics on BR are split. Total Zone Rating has him a bit below average; the Baseball Info System measurements (runs saved and plus-minus) solidly above average.
The Sabr Defensive Index combines five metrics (including Total Zone and runs saved), so it's not an independent, reinvent-the-wheel attempt to measure defensive prowess. It's more an attempt to find consensus among the existing metrics.
Everybody who does this kind of analytics is quick to add cavets, and one of them is small sample sizes. The SDI list above is just a half season. But Mauer, I think, is becoming a good defensive first baseman. It's just taken longer than I expected when he changed positions.