ST. LOUIS -- When the Boston Red Sox signed Mike Napoli this past winter, there were a number of possible outcomes. He could hit a little, the way he had in 2012, putting up an OPS+ of 113. He could hit a lot, like he had in 2011, with a 173 OPS+ and 30 home runs. Most likely, he'd fall somewhere in between.

Then there was the question of his health. A degenerative hip condition reduced his free agent contract with the Red Sox from three years, $39 million to a single year and $5 million once the problem was discovered. Moreover, the longtime catcher had to find another position. Like, say, first base.

You know how this goes, though. The former catcher, limited mobility, goes to first base as a placeholder. Maybe he takes some time at DH to rest, but when he's at first, it's as a placeholder, the way the Red Sox have deployed David Ortiz there in the World Series games played at Busch Stadium.

Amazingly, though, that's not what happened with Napoli. He's actually been great. And by winning Game 4 in St. Louis Sunday night, the Red Sox guaranteed themselves more than just one more home game this series -- they'll also get Napoli, among the most valuable defensive first basemen in baseball this year, back on the field.

"What we've seen, in addition to a good athlete who plays low to the ground, with good hands, the way he's worked around the bag, with a lot of throws in the dirt, he has saved a number of runs," Red Sox manager John Farrell told me about Napoli prior to Game 3 on Saturday. "We're ecstatic with the way this year has turned out for him defensively."

It's important to note that Napoli had played first base prior to this season. But he hadn't played it much, and he hadn't played it all that well (with the caveat that defensive metrics can be unreliable over a single season).

Napoli logged 586 1/3 innings at first for the 2010 Angels, putting up a UZR/150 just above average. But he slipped to -8.3 UZR/150 in 246 1/3 innings at first base for the 2011 Rangers, and -14.4 UZR/150 in 207 2/3 innings there for the 2012 Rangers. To put that in perspective, 53 major league first basemen logged at least 200 innings at first base in 2012. Napoli's season placed him 49th among that group.

So how, then, to figure out his 2013? Among first basemen with 200 innings at the position, Napoli ranked fourth of 54. But the three ahead of him, Josh Satin of the Mets, Daric Barton and the Athletics, and Kyle Blanks of the Padres, each played fewer than 300 innings at first this season. Napoli played just under 1,100. By the numbers, he was one of the best regular defensive first basemen in baseball this year. And he's done it at age 31, with a pair of degenerative hips, playing roughly as many games at first this year as he'd played, total, in his major league career.

"It is unheard of," Red Sox third base coach Brian Butterfield said of the level of defensive proficiency Napoli has achieved.

So how'd this happen?

"It tells you that his work ethic and his athleticism are well beyond what maybe some had envisioned," Farrell said. "The work that he's done with [Brian Butterfield] from day one of spring training, the work that they put in is the exact reason as to why he's become the defender he's been. He's a good athlete, but there was a lot of work put into that."

Amazingly, though, Napoli had to undertake that work while treating the hip condition, which limited his work early on in the spring. Butterfield, a minor league infielder who serves as primary coach around the diamond, didn't know what to expect when given the task of making Napoli into a usable first baseman.

"Well, it was a little tough at first," Butterfield, who looks like an earnest John C. McGinley, told me as we chatted Saturday afternoon in front of the Red Sox dugout. "Because in spring training, he had restrictions, from the physicals he'd had in the winter. So we were on a limited work plan in spring training. But it was obvious, once he got going, got the green light, it was obvious he wanted to work."

That work started even before Napoli got that green light.

"When he was limited, we put him on his knees, and just did hand work drills," Butterfield explained. "So he would get on his knees, with his glove, and we'd empty a bucket of ball, I'd bounce 'em at him, he'd just reach with his hands."

But a week into spring training, doctors gave him the go-ahead to play, and Farrell, eager to see what he had, played him regularly at first. Still, though, that process had to be gradual.

"He was able to do stuff on his feet, but it was limited," Butterfield said. "First it was 25 ground balls, next day it was 30. And then we finally got to a point where he had a full workload."

The strategy of playing him so often in spring training had as much to do with maximizing what he could be defensively as seeing what the Red Sox had.

"John played him a lot in spring training, and that helped us evaluate him," Butterfield said. "And that gave us a lot of confidence heading into April, what we saw in spring training.

"He's continually gotten better at first as he's gotten game repetitions," Butterfield said. "I think that's the key. You can work a guy off the end of a fungo bat for six months. But it's just not the same as being in live game situations, where you have to move, you have to chase the ball, you have to go to the bag to catch throws on the base. The more reps he got, the better he got."

Meanwhile, Napoli's offense thrived as well, with a 129 OPS+ serving the Red Sox well out of the first base spot. But then a funny thing started to happen: Mike Napoli, signed for his bat, was used as a defensive replacement after pinch-hitting in the seventh inning of a May 30 game against the Phillies. On July 14, and again on August 7, he didn't even pinch-hit, was simply brought into the late innings of a game for defense. It happened twice more in August, and three times in September.

And it got to the point, in September, where Butterfield actually had to dial back Napoli's work at first, just to make sure he had fresh enough legs to hit, too.

"He's a big guy, and we want him to be an offensive guy," Butterfield said. "I limited him on ground balls in September. As much as he wants to work, I'd tell him, 'You're gonna take five ground balls today, and that's it, so we can have some fresh legs in September and [beyond]. He's done enough where we feel really good about everything he does defensively."

That's more than many other first basemen, thanks in part to the defensive schemes employed by the Red Sox. If not for Napoli's emergence, the Red Sox simply wouldn't be able to do nearly as much shifting.

"Against right-handed hitters, against Mark Reynolds, against Jhonny Peralta, early in the year, we would play three guys on the left side. Which means, the further the first baseman can get off the bag, the more he's gonna help us. So as Mike became more comfortable with moving away from the base, the more we were able to overshift."

The rest of the league hasn't necessarily caught up yet to what the Red Sox coaches knew they had back in March.

"It was about midway through spring training," Butterfield said. "He'd made enough plays when we just looked at each other, when we're all in the dugout, and said, 'Okay. We've got ourselves a first baseman.' You could tell."

Still, Napoli didn't manage to earn even one of the three American League first base finalist slots for the Gold Glove this season. That's something both Butterfield and the metrics find mystifying.

"Honestly, I don't know who won the Gold Glove, but I've heard that Mike wasn't the winner," Butterfield said. "But they had to be awful darn good to beat, to have a better year than he had defensively. And in my mind, he was the American League Gold Glove first baseman."

So by now, no one should have been surprised it was defensive replacement Mike Napoli making the tag on Kolten Wong to end Game 4 Sunday night. Nor was it an enormous stretch that shortly after Butterfield and I first talked, Napoli took some ground balls at third base, though Butterfield assured me in a subsequent conversation that it was only for an emergency.

Why not, though? The Red Sox already know Napoli can not only learn a new position adequately, but excel at it. Napoli's price this winter is likely to reflect it, with many teams well aware of the defensive skills overlooked by the Gold Glove finalists.

Still, I wondered: Was this the likely outcome in Butterfield's mind last winter?

"Not last winter, before we had gotten him," Butterfield said, chuckling. "But a credit to Mike, as hard a worker as he is, and as athletic as he is, it's an unbelievable compliment to him."

And for at least one more game this year, it is an added dimension for the Red Sox.