Late last night the Cubs finally landed pitcher Jon Lester, arguably the highest-profile free agent on the market, for a six-year, $155 million deal, or $25.8 million per year. It's a huge signing, signaling that the team is ready to contend next year rather than let their impressive young talents develop en route to 2016.

Jon Lester is really good. But a quieter acquisition could mean just as much for the team's pitching. That's veteran catcher Miguel Montero, just picked up from the Arizona Diamondbacks. And it says a lot about how the Cubs are approaching their rebuilding.

By the top-line offensive statistics, Montero and current catcher Welington Castillo had comparable years in 2014:

  Batting Average On-Base Percentage Slugging Percentage Home Runs
Castillo .237 .296 .389 13
Montero .243 .329 .370 13

Montero has hit well in the past, but he's had two straight years of mediocrity at the plate. Meanwhile, Castillo hit better than Montero in 2013, when Montero was awful, and Castillo is four years younger and way cheaper. He's under team control until 2018 and expected to make $2.1 million after arbitration. Montero's due $12 million next year, and another $14 million in 2016 and 2017.

Even assuming that Montero is a somewhat better hitter than Castillo in the immediate future, why would he be so much more valuable than Castillo?

The short answer: pitch framing. Montero is very good at it. Castillo is not. And it makes a much bigger difference than you might think. Pitch framing is simply a catcher's ability to get a strike call on a borderline pitch, and teams are increasingly attuned to it as a skill.

By StatCorner's data, Miguel Montero was the best at it in baseball during 2014, saving 24 runs above average last year with his framing skills. Welington Castillo was the second-worst, giving away 24.3 runs above average. By Baseball Prospectus's data, Montero was ninth-best, and Castillo was sixth-worst.

Quantifying pitch framing is tricky, so the two sources aren't in sync. But they're both in agreement that Welington Castillo was one of the absolute worst pitch-framers in baseball for the past two years. Both agree that Montero's performance has moved around a bit, varying between above-average and great.

To make things simple, let's use the Baseball Prospectus career statistics for Castillo and Montero's pitch-framing abilities.

Castillo: -11.1 framing runs per 7,000 framing chances
Montero: 8.7 framing runs per 7,000 framing chances

And let's use 8,000 chances, about what Castillo had last year, for comparison.

Castillo: -12.7 framing runs
Montero: 9.9 framing runs

So that's a total swing of 22.6 runs.

Now, let's look at Jon Lester. He was great last year, with a 2.46 ERA in 219 innings. His fielding-independent pitching—sort of like ERA, only trying to isolate the pitcher's performance from his defense—was, according to Fangraphs, 2.80.

Lester's 2014 was his best year in a long time. But there's evidence he took a different approach, so the change could be permanent and not a matter of luck. So to make things simple, let's compare Lester's fielding-independent pitching last year to the league-average FIP of 3.81, and assume he throws 219 innings (a career high).

League-average starter: 93 runs
Lester: 68 runs

The difference? 25 runs. That's very close to what Montero would theoretically save them.

Here's another way of looking at it, from Fangraphs:

The numbers agree Montero is about a +2 WAR framer. Let’s cut that in half. Add it to his optimistic +3 WAR Steamer projection, and Montero comes out as something like a +3.5 WAR catcher in 2015. The numbers agree Castillo is something like a -15 framer, let’s regress that to -5. Castillo comes out as something like a +1.5 WAR catcher. It isn’t a monumental upgrade, but it’s an upgrade.

So, conservatively, Montero is worth two wins-above-replacement more than Castillo, if you include pitch framing. The difference between the Cubs' innings-leader last year, Travis Wood, and Jon Lester's Fangraphs projection for 2015? 2.4 wins-above-replacement.

As noted, Lester might be a better pitcher than his projections for 2015, since he's arguably made improvements to his approach (he was worth 6.1 WAR last year). Then again, in the excerpt above, the difference between Montero and Castillo is given a pretty conservative estimate.

But a lot of things could happen, including a Montero/Castillo platoon that would play to their offensive strengths; Montero could get the bulk of the starts against right-handers, whom he hits well, and vice versa. Either way, the whole point of this exercise isn't to nail down the inevitably unpredictable numbers, but to show that acquiring Montero could improve the Cubs' staff as much as acquiring a good pitcher—perhaps not as much as acquiring Jon Lester, but within the same ballpark.