The Boston Red Sox were an excellent team in 2017, going 93-69 and winning the American League East Division before losing to the Astros in the Division Series.
The Red Sox made a major transaction in the offseason, signing JD Martinez as a free agent. As you know, Martinez has terrified pitchers all season, hitting .333 / .400 / .665 while leading the majors in home runs (37), RBIs (104), extra-base hits (71) and total bases (292). He is second after teammate Mookie Betts with a .333 batting average, 17 points behind Betts, so a Triple Crown is still a possibility.
As you also know, the Red Sox have terrorized the entire American League throughout the season. They have won five in a row after beating the Phillies 2-1 on Brock Holt's home run in the eighth inning on Tuesday night, are 18-5 from the All-Star Game and are about to finish 115-47 - - meaning they have set the record for the 2001 Mariners' 116 wins and 1906 Cubs at stake.
It's remarkable that an already excellent team demonstrates this kind of improvement, so here's the follow-up question: How much credit does Martinez deserve for making the Red Sox one of the best in a single season in MLB history? ?
The Red Sox have scored 661 runs through 121 games; At last year's rate, they would have scored 587 runs in 121 games. There are 74 more races, but Martínez is not responsible for the 74 races. We can estimate that Martínez has created approximately 114 races so far; Last year, the Red Sox DH created about 97 runs (or 72 out of 121 games). Martinez has created approximately 42 more runs so far than last year's DHs. (For this quick and dirty little study, we'll ignore Martinez having started 44 games in the outfield. We're trying to figure out how much of the offensive improvement should go to Martinez.)
Others deserve credit, especially Mr. Betts. After all, Martinez doesn't even lead his own team in on-base or slugging percentage; Betts does. Betts is hitting .350 / .439 / .668 compared to .264 / .344 / .459 in 2017. He has already created more runs than in all of 2017: around 124-105, via the numbers in Baseball-Reference .com. Prorated over the same number of plate appearances, Betts has been an incredible 55 runs better.
In fact, Betts' improvement may have had a greater impact than Martinez's addition.
Of course, that ignores the spill effect of adding Martinez. Maybe he provides alignment protection. Maybe he makes the other hitters better just because of his presence. Those are speculative ideas, however. One of the oldest and longest-running sabermetrics issues is the issue of alignment protection, but extensive study after extensive study has failed to demonstrate that such a thing exists. The Red Sox are better at the plate because Martinez is better than the players they had last year in their positions and because Betts is better than Betts.
All of this ignores launch and defense. The Red Sox allowed 3.69 runs per game compared to 4.12 in 2017 (52 runs better in 121 games). Even adjusting for the slight decline in offense throughout the league, pitching and defense have been better than last season.
Martínez has been incredible. It has been worth 5.6 WAR so far, in pace for a 7.5 WAR season. That is a season at the MVP level, and his push from the Triple Crown places him alongside José Ramírez, Betts, Francisco Lindor and Mike Trout in the MVP career. If you go deeper, you can also find some added value on the part of Martínez, for example, he leads the majors in the probability of winning aggregate. He released .368 with runners in scoring position. He threw .326 / .442 / .651 in "late and close" situations and .360 / .462 / .748 when the game is tied. It really is purged in every situation.
The Red Sox can finish 22 games better. Martinez can be worth seven to 10 of those wins. And he may not even be the most valuable player on his own team.
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