Chris Sale is going to improve an already-dangerous Red Sox roster simply because he's one of the best pitchers in baseball and he'd improve any team significantly. But what if the opposite effect is true, too? What if simply moving to the Red Sox could improve Sale?
Though Fenway Park isn't necessarily the most friendly home for lefty pitching, there's an enormous reason why Sale ought to be happy to be in Boston, and it's not just the presence of elite fielders like Mookie Betts and Jackie Bradley Jr. behind him, though that certainly helps. It's because the 2016 White Sox catchers were among the poorest pitch-framing groups on record. Simply by getting away from the South Side, and to Boston's more efficient catchers, Sale could look like a more effective pitcher.
"Pitch framing," for those not familiar, is the ability of a catcher to receive a pitch in a way that he makes it more likely an umpire will call a borderline pitch a strike. Though it's been known in the game forever, it's only had the ability to be quantified in recent years, and we're seeing teams value it. The Twins just gave $24.5 million to Jason Castro, despite the fact that he rarely hits, because he's a quality framer. The Diamondbacks just gave two guaranteed years to Jeff Mathis, despite the fact he never hits, because he's also a quality framer. It's an important skill.
It's also a skill the 2016 White Sox simply did not have, as Alex Avila and Dioner Navarro ranked extremely poorly in pitch-framing metrics. Looking at Baseball Prospectus' pitch-framing metrics, which considered 114 catchers this year, Navarro ranked 114th, costing his pitchers -16.8 runs. Avila was 100th, at -6.8 runs. For an idea of the spread, the two best framers, Buster Posey and Yasmani Grandal, were in the +27 runs range. To add insult to injury, former White Sox catcher Tyler Flowers was sixth-best, at +12.3 runs for Atlanta.
Flowers caught every pitch Sale threw in 2014 and '15 and his departure from Chicago  "blindsided" Sale, who had clearly developed a special bond with the catcher. How big a deal was the change in Chicago backstops? Well, we can look at framing not just in terms of what catchers provide, but how pitchers are affected. Again looking at Baseball Prospectus, which for the 2015 season ranked 807 pitchers, Sale was essentially tied with Zack Greinke (who was paired with Grandal) for the most framing help in baseball, receiving +10.7 runs of help.
Your takeaway from that paragraph should be "in 2015, Sale was helped by his catcher as much as anyone." Now, let's look at the 2016 rankings, which included 824 pitchers, and, well ... you probably already know that this is not going to end well.
Worst pitch framing support from catchers, 2016
-7.8 runs -- Brandon Finnegan, CIN
-6.3 runs -- Sale, CHW
-5.3 runs -- José Quintana, CHW
-5.1 runs -- Hisashi Iwakuma, SEA / John Lamb, CIN
As Jeff Sullivan of FanGraphs pointed out, the ensuing drop of 17 runs of support was the single largest drop in baseball. That's two White Sox pitchers in the top five, and if we'd gone to 10, you'd have seen Carlos Rodón, too. Every time the Chicago catchers didn't help their pitchers, they were putting them at a disadvantage.
What does that look like? Well, compare what happens when you give one elite lefty starter (Sale) below-average framers against what happens when you give another elite lefty starter (Madison Bumgarner) a top framer in Posey. The image below overlays the pitches outside of the zone that became strikes (orange) with the pitches inside the zone that became balls (blue). It shouldn't be hard to see who got more help.

But Sale is free of all that now. In Boston, Christian Vázquez is considered to be a plus framer, adding +7 runs in 2016 despite playing just 57 games. Sandy Leon (-4.5 runs) is slightly below average, while Blake Swihart (-0.7 runs) was about average in an injury-shortened season. The point is not that Sale is suddenly going to get the same boost that Flowers once offered, though it's possible if he's paired consistently with Vazquez. It's that he's no longer going to be as limited as he was in Chicago. Even Leon is miles better than Avila or Navarro.
You could see the effects, anyway. In 2014, Sale whiffed 30.4 percent of the hitters he faced. In 2015, still with Flowers, that was 32.1 percent. In 2016, that dropped to 25.7 percent. The effects of a count going from 1-0 to 0-1 have been well-studied, and for the first time, Sale had those effects seriously working against him.
He's so good, of course, that it might not matter who he throws to. He was still great in 2016, after all. But even great pitchers need help. Sale didn't get it last year. He'll get more next year. That's not great news for the rest of the American League East.