Last off-season, when the Red Sox lost the services of Jason Varitek, they ended up signing former Boston farmhand Kelly Shoppach to replace him behind the plate. Shoppach was expected to hit if platooned properly, now that he would be in Fenway Park rather than the offense killer that is Tropicana, but defensively... well, there were questions.
Shoppach's percentage of baserunners thrown out spiked while with the Rays, in part because he was catching James Shields, who not only led the league in picked off runners in 2011, but also was absurdly effective at holding them at first and shortening their leads and jumps. The thing is, while that's the part of the defense game that's most obvious, it's also just a small part of the equation. Overall, Shoppach just wasn't -- and isn't -- a very good defensive backstop, and Boston was going to take a hit there given Varitek's previous contributions.
Varitek was excellent defensively, even in his later years, if you follow the story told by advanced metrics for catchers. You should, too, as the gains being made in that specific part of the defense discussion are landing multiple analysts jobs with major-league teams who are as hungry for answers as those they bring on board. Even as Boston's backup, he was among the league leaders in defensive runs behind the plate, leading one to wonder just how valuable he was back when he was younger and catching the majority of games for the Sox. Replacing that kind of greatness behind the plate with someone like Shoppach, who is essentially replacement level because of his glove, likely would have been more of a story in 2012 had so many other things not been going wrong with the club.
Jarrod Saltalamacchia has a bat that helps carry the fact he's basically an average defensive catcher. Saltalamacchia frames pitches well, but he isn't great at blocking them* -- essentially making him the opposite of Varitek, who could block like no one else but wasn't much for framing. In reality, you can only really be great at one or the other, as they come from two totally different backgrounds. Framing sees a catcher setting up higher in the strike zone, on the corners, while someone who wants to utilize his ability to block, as Varitek did, is going to have pitchers burying hard sinkers, cutters, breaking balls, and so on, out of the batter's reach but into the catcher's mitt. So, it's not a bad thing that Salty isn't Varitek, but it does mean the pitchers work a bit differently, and could be part of all those whispers of certain pitchers preferring Varitek over Salty, or others not minding one way or the other.
*Let's not forget, though, that the fact Saltalamacchia had to catch Tim Wakefield didn't exactly bolster his blocking numbers, either. He's likely better at that than the advanced metrics say, even if he isn't great at it.
Saltalamacchia isn't really the problem defensively, though, even if the Red Sox wish he were better at it. The problem in 2012 was Shoppach, who isn't good at framing, doesn't contribute much to pitch blocking, and, while still solid at throwing out runners, fell back a bit from 2011's pace after his separation from Shields. According to Max Marchi, Baseball Prospectus' current advanced catcher defense guru, Shoppach was a win below replacement level despite only playing in 48 games. That's not good, but when you combine it with the loss of Varitek, who had been worth multiple wins in limited duty the year prior, it looks even worse.
This is a long way of explaining why it is that David Ross is more important than he might have initially seemed to Boston. Ross can hit, and better than Shoppach. He can field, too, better than anyone Boston has had since Varitek hung up his mask and chest protector. In 2012, using Marchi's data once more, Ross was worth something in the neighborhood of three or four wins above replacement, putting him -- in just around 60 games time -- roughly five wins better than Shoppach. That is a gigantic swing, the kind of improvement that teams pay seven figures for on long-term deals with superstars, but since catcher defense at this point isn't as universally understood and valued as your more basic areas of the game, it's currently something teams can acquire on the cheap. Ask the Rays, who have Jose Molina behind the plate, or the Brewers, who made sure to lock Jonathan Lucroy up to a long-term deal, or even the Sox, who succeeded for so long with Varitek behind the plate, even as his bat's effectiveness waned.
Now, that doesn't mean Ross will be a five-win improvement. He's a year older, and success in the past doesn't automatically mean the same success in the future. However, if he hits well once more, and he brings the defensive value to Boston that attracted the team to him in the first place, then whatever value he does provide will be significant when compared to last year. And this team is all about making people forget about last year.