The conventional wisdom around these parts is that John McDonald is a wonderful gloveman, and Eckstein stinks. The naked eye supports this, and (Canate told me on the phone tonight) so do the Baseball Prospectus writers. (Note: PECOTA can still blow me, after it finishes with Dr. K).
But let's take another look, using BP's own advanced stats. Bear in mind that Eckstein and McDonald are both 33 this season. According to BP's RAA2 (Fielding Runs Above Average, adjusted), over the past 4 years McDonald has registered 3, 6, 6, and 16. Over that same period, Eckstein scored -12, 11, 19, and -10.
What does that tell you? Honestly, it doesn't tell me shit, other than that fielding ability fluctuates a great deal year-to-year, and that it remains insanely difficult to measure. Mcdonald probably was a lot better than Eckstein with the glove last year, but he was also much better (at age 32!) than he had himself been in the past. What are the odds that he will be as Ozzie Smith-like going forward? I'd say they're slim. Players probably have career seasons in the field, just as they have career seasons at the plate. And it's likely we witnessed JMac's version of a Brady Anderson 50 in '07.
I'm not saying McDonald can't be a useful cog on this (totally awesome) team, or that he shouldn't form half of an offense/defense platoon with Eckstein. I'm just saying that the argument that he should get 140 starts, with Eckstein on the bench (despite giving up -- according to the career #s -- 80 pts of OBP and 50 pts of SLG to the scrapper), is not automatic.