Most Pirates fans, in their heart of hearts, know the answer to that. They saw how crucial McCutchen was to the Pirates' summer surge -- he posted OPSes of 1.096 or higher in May, June and July as the Pirates climbed to a high point of 16 games above .500. He showed more power than even Pirates fans expected, hitting homers to all parts of the park. Then the fans saw how McCutchen vanished in August, posting a .693 OPS. McCutchen was hardly the only Pirate to pull a disappearing act around that time, of course, but his cold streak hurt them as badly as anyone's.
That's not to pin the Pirates' downfall on McCutchen, of course. He's their best player, and he, more than anyone, helped put them in a great position until the late summer. And with other wild card contenders falling apart around them in August and September, the Pirates could have still made the playoffs despite McCutchen's bad month if only more of his teammates had taken care of business.
Besides, when determining the league MVP, what matters shouldn't be a player's trajectory through a season. It's his value over the season as a whole. But while McCutchen played like a clear MVP through July, his mediocre August bumped him down the ranks.
Fangraphs ranks Posey first among NL position players in wins above replacement, with 8.0. Braun is second, with 7.9. David Wright and Headley are third and fourth. McCutchen is fifth, with 7.4.
McCutchen's low ranking is due in part to his defense, which advanced metrics don't much care for. You may remember that McCutchen won a Gold Glove award last week. That award was a farce. McCutchen isn't even an above-average centerfielder. He wasn't even the best centerfielder on his team last year, or the second-best. (Those would be Starling Marte and the now-departed Gorkys Hernandez.) McCutchen has a weak arm and he doesn't do well on balls he has to go back on. As a left fielder, he'd probably be terrific. As a centerfielder, not so much. In the end, McCutchen had a great hitting season while manning a tough defensive position. He provided tremendous value to the Pirates, but this year, it's not enough.
Instead, I would probably go with Buster Posey, who had an offensive season that was broadly similar to McCutchen's, finishing slightly ahead of McCutchen in batting average and on-base percentage and just behind him in slugging percentage. Posey played catcher, an even more crucial position than center field, and played it well, providing more defensive value than McCutchen did. (He threw out 30 percent of opposing basestealers, a figure Rod Barajas and Pirates fans can only dream about.) In the end, the voters will likely pick Posey over McCutchen as well. After all, Posey stayed hot while McCutchen slumped down the stretch, and the Giants ended up winning the World Series. (Posey had one bad month, just as McCutchen's did, but Posey's was in May rather than August.)
McCutchen had a brilliant season, of course. It just wasn't quite an MVP-caliber season. He just turned 26 last month, so he'll have plenty more shots at MVP awards. I would bet that he'll win at least one before his career is over. Unfortunately, he probably will, and ultimately should, have to wait.