There are many big deals to be made at this week’s winter meetings, but sometimes the best deals are the small ones -- the ones you barely notice on a random winter Friday while pining for spring to get here ASAP. Mike Hazen’s most recent move in his first winter as Diamondbacks general manager was to non-tender everyday catcher Welington Castillo and sign former Angels and Marlins catcher Jeff Mathis to a two-year deal worth $4 million.

Mathis hit .238 with two home runs in 126 at-bats last season. He turns 34 just before Opening Day. His slash line over 12 major-league seasons is .197/.254/.308. His .562 OPS is the lowest of any active player with at least 1,000 plate appearances. Mathis doesn’t have a high Q rating either. His Twitter following barely clears 1,500.

So what makes the Diamondbacks signing the underappreciated Mathis a big deal?

“His defense has always been his calling card,” one major-league scout said. “His hands are excellent. His arm strength and accuracy are above average. He blocks pitches in the dirt well. He handles pitching staffs well and fulfills the pitching coach’s game plans ideally.”

The numbers back that up. Over the past nine seasons, Mathis ranks fifth among catchers in defensive runs saved. And on a per-inning basis, he rates better than the leader -- Yadier Molina.

How Mathis impacts a season

Speaking of rating on a per-inning basis, there’s another important thing to note: Mathis rates very well in pitch-framing in that context. That the Diamondbacks went for an elite pitch-framer to fill one of their two catching spots isn’t surprising for two reasons. First off, six of the top eight teams in the stat that measures pitch-framing (which we'll get too shortly) made the 2016 postseason, and success begets impersonation. Second, the announcement of Mathis’ signing came not long after the announcement of the hiring of Pirates quantitative analyst Mike Fitzgerald as the team’s director of research and development.

Fitzgerald also pounded the table for Russell Martin, and his pitch framing, in the 2012-13 winter. That signing was not popular...at first

— Travis Sawchik (@Sawchik_Trib) December 3, 2016

The Pirates' success the last few seasons can partly be traced to their prioritizing those catchers who rated highly at pitch-framing: Russell Martin, Francisco Cervelli and Chris Stewart (as Sawchik documents).

Castillo brought a good bat. He hit 31 home runs in 690 at-bats and produced 3.8 WAR over two seasons with the Diamondbacks, but his WAR didn’t factor in his pitch-framing, which was better in 2016 than in previous seasons but has consistently rated poorly throughout his career.

Mathis is consistently very good to excellent, frequently rating in the top third of catchers in that stat. In 2016, Mathis ranked fourth in the majors at “called strike rate above average” (in other words, how often was he getting called strikes on pitches compared to his peers) behind Buster Posey, Yasmani Grandal and former Diamondbacks catcher Miguel Montero. The Diamondbacks have ranked 28th in that stat over the past two seasons, which coincides with the departure of Montero.

As the image below shows, Mathis was excellent at getting his pitchers strike calls on low pitches, inside pitches and outside pitches. He’s said by those around baseball to have what are called quiet hands, meaning he can make borderline pitches look like strikes without making it too obvious. He rated best in baseball at getting pitches in the lower-third of the zone (or below the zone) called a strike in 2016.

How Mathis impacts a pitcher

Congrats 6. Thank you for all you did for me the last 4 years. AZ is getting hell of a teammate. Your presence will be missed.

— Tom Koehler (@TKREFRESH22) December 3, 2016

On the surface, those are nice words from one former teammate to another. But there’s more to it than that. Through 711⅔ big league innings, Tom Koehler is a below-average starter. He’s not a bad pitcher, but his 4.16 ERA is a bit higher than average and his strikeout-to-walk rate isn't great.

But last season, one in which Koehler tinkered with his delivery midyear, the Koehler-Mathis connection was special. In 13 starts with Mathis behind the plate, Koehler looked like a Cy Young contender. His ERA and strikeout-to-walk rate were both 2.65, and opponents hit .191 with a .593 OPS against him. In 20 other starts, Koehler had a 5.66 ERA, and his strikeout-to-walk rate was 1.4; opponents hit .309 against him.

Admittedly, you can get into trouble with these sorts of comparisons, because they don’t factor in caliber of opponent (and those 13 games included three against the offensively challenged Phillies). But let’s at least give Mathis a piece of the credit for putting Koehler on a path to success. From a pitch-framing perspective, there’s this: In Koehler’s career, Mathis has gotten him 12.6 called strikes more than the average catcher would have gotten. All other catchers are at a combined minus-78.3

As it applies to the Diamondbacks, our thinking is this: If Mathis can be a part of the success of a pitcher like Koehler, isn’t it reasonable to foresee that he could be a part of the return to form of Zack Greinke, Shelby Miller, Patrick Corbin or anyone else who was a part of the 2016 team, whose 5.09 ERA was worst in the majors?

Mathis won’t be an everyday catcher (offensive-minded Chris Herrmann may be), but he fills a vital need.

The bigger picture

Friday was a busy day in baseball and a notable one for pitch-framers. The Nationals traded for Derek Norris, whose strikes looking above average numbers were very good for the Padres the past two seasons. The Mets re-signed Rene Rivera, who is regarded highly for his pitch-framing and work behind the plate, his NL wild-card game notwithstanding. Castillo was non-tendered, as was Reds catcher Ramon Cabrera, who didn’t hit well last season and who ranked last in that pitch-framing metric.

As Buster Olney pointed out two weeks ago, the Jason Castros of the world who bring value with framing are getting three-year contracts, and the Matt Wieters types who struggle in that area are having to hunt for suitors.

So you may not have thought much of the Mathis signing, but it will be one to look back on during October next year. See if the Diamondbacks pitchers think it's a big deal.