Heading into Sunday's series finale at Citi Field, Murphy was hitting .418 (23-for-55) with seven homers and 21 RBI in 15 games against his former club in 2016.
Just how historic is that production? The last player to drive in more runs in his first season facing his former employer was Roger Maris. He had 27 RBI against the Kansas City Athletics in 1960 after getting dealt to the New York Yankees the previous offseason.
Murphy also has a shot at having the best home run and RBI totals ever in a season against the Mets. Willie Stargell in 1966 and Dick Allen in 1968 produced the most homers against the Mets with 10 apiece. Hank Aaron in 1962 produced the most RBI ever against the Mets with 28.
After completing this weekend's meeting on ESPN Sunday Night Baseball, the Mets and Nationals have one more three-game series in D.C. from Sept. 12-14, during which Murphy can pad his stats.
"Obviously we're not making quality pitches, because he's done huge damage," Mets manager Terry Collins said. "I'll tell you, I applaud him. I really do. Last year, when he became a free agent coming off the postseason that he had, I was shocked that it took so long for somebody to get him signed. I think Dan Murphy -- not only just against us, but against everybody -- said, 'Look, I'm a good player, and I'm going to prove it,' and got himself in tremendous shape. I thought he was in great shape in spring training. And it's carried over into the year he's had. We've just got to make better pitches."
Murphy's production really is a continuation of his historic October, when he homered in an MLB-record six straight postseason games and was named National League Championship Series MVP as the Mets swept the Chicago Cubs.
So why precisely did the Mets let him walk as a free agent? Here's a breakdown of potential reasons:
Single dimension: Murphy is hitting .340, trailing Colorado's DJ LeMahieu by five points for the NL lead. Still, while Murphy's average exceeds most projections, the Mets and everyone else knew Murphy could hit for average. The Mets' view on Murphy, however, was that he was a one-dimensional player who could hit for average -- and doubles -- but not much more than that.
Yes, Murphy flashed power late in the regular season and during the postseason after Mets hitting coach Kevin Long impressed upon him to become more pull-conscious rather than be content slapping the ball to the opposite field for singles. But the Mets did not believe the power would be sustained at its current level. Murphy has a career-high 25 homers. His previous best was 14 last season with the Mets.
Also, despite his success, Murphy really doesn't subscribe to the Mets' approach at the plate. He averaged 3.5 pitches per plate appearance in 2015 -- the lowest rate on the team, at least once pitchers are excluded. The league average was 3.8 and the Mets' average was 3.9.
And while the Mets don't place a high value on defense, clearly Murphy was not an asset in the field or running the bases while a Met. After one Murphy misadventure on the basepaths, David Wright joked that Murphy "sometimes thinks that he's invisible."
One Mets broadcaster used the term "net negative" to explain why the Mets ought to let Murphy depart. That phrase is now frequently thrown around on Twitter by frustrated Mets fans watching Murphy's production for a rival.
Heir apparent: This reasoning looks silly now, but at the time the Mets viewed Dilson Herrera as the heir apparent to Murphy. So it made little sense to make a longer-term commitment to a player entering his 30s with a history of injuries. Of course, Herrera ended up getting traded to the Cincinnati Reds for right fielder Jay Bruce on Aug. 1 after a previous package highlighted by outfield prospect Brandon Nimmo unraveled earlier in the day.
The Mets dispute it as a factor, but Herrera's MLB minimum salary relative to a $12 million-plus per year commitment for Murphy had to be some factor in letting him walk. After all, the Mets still are payroll-conscious, despite things relaxing somewhat. And their stud young pitchers are beginning to enter their arbitration years, when their salaries will begin to soar.
Off the field: Let's be fair: The Mets made Murphy a one-year, $15.8 million qualifying offer. So it was not as if they wanted Murphy out at any price.
Still, while a minimal factor at best, Murphy's publicly articulated personal views might have made it easier to justify his departure. For instance, Murphy expressed his disapproval of gay marriage, which generated unwanted headlines.
The bottom line: In retrospect, the Mets clearly made a mistake letting Murphy depart. After all, he received only a three-year deal elsewhere, Herrera has been traded and replacement Neil Walker is now lost for the season because of a herniated disk (albeit after clubbing a career-high matching 23 homers).