Take a look at Napoli's past four seasons, the four in which he's played more than 100 games:
2009: 114 games, .272/.350/.492, 2.9 WAR
2010: 140 games, .238/.316/.468, 2.8 WAR
2011: 113 games, .320/.414/.631, 5.6 WAR
2012: 108 games, .227/.343/.469, 2.0 WAR
Wins Above Replacement is a counting stat that's also highly context-dependent. So the number of games played, and the number of games played at a defensively challenging position like catcher (as opposed to first base or DH), makes a big difference. Brushing all that aside, though, it's tough to imagine Napoli ever again replicating his 2011 production. That year, he posted a career-high .344 batting average on balls in play (.299 lifetime) despite a line-drive rate that wasn't far out of line with career norms. He benefited from playing in hitter-friendly Arlington, but even that doesn't fully explain a season in which he was the best hitter in all of baseball.
Last season wasn't your typical regression to the mean, either. Napoli hit a career-low .227, posting a career-high 30 percent strikeout rate. A lefty-masher throughout his career (.911 career OPS vs. LH), he hit just .179/.295/.411 in 2012. Check out this hot zone map from ESPN Stats & Info — it's one big cold zone, basically.
The good news, aside from the Red Sox talking Napoli down from his initial demand of four years, is that he'll now be freed from major catching duties and will serve primarily as the team's first baseman. We still don't have definitive studies on how much a slugging catcher benefits when he's relieved of the burden of catching. But Napoli has had some injury problems in the past, he's 31 years old, he's not as mobile as he used to be (hitting only nine doubles all last season is something of a statistical fluke, but still) and there's a long history of catchers breaking down by the time they get to their mid-30s. Yes, we all know that Napoli has a career 1.107 OPS at Fenway, but that is over only 73 career plate appearances and shouldn't be taken all that seriously. He'll be a below-average defender at first, but if he can play 140 games, with half of those in front of the Green Monster, after a down season in which he still whacked 24 homers, another 30–home run season is still a real possibility.
This is a reasonable deal for a righty-swinging slugger with some bounce-back potential. If the Sox can flip Jarrod Saltalamacchia or Ryan Lavarnway for help elsewhere (they also have David Ross, and Napoli still figures to catch at least some games), all the better.