The Boston Red Sox were an excellent team in 2017, going 93-69 and winning the AL East before losing to the Astros in the Division Series.
The Red Sox made one major transaction in the offseason, signing J.D. Martinez as a free agent. As you know, Martinez has terrorized 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's second to teammate Mookie Betts with a .333 batting average, 17 points behind Betts, so a Triple Crown remains a possibility.
As you also know, the Red Sox have terrorized the entire American League all season. They've won five in a row after beating the Phillies 2-1 on Brock Holt's eighth-inning home run Tuesday night, they're 18-5 since the All-Star break and they're on pace to finish 115-47 -- which means they've put the record 116 wins of the 2001 Mariners and 1906 Cubs in play.
It's remarkable for an already excellent team to show this kind of improvement, so here's the follow-up question: How much credit does Martinez deserve for turning the Red Sox into one of the greatest single-season powerhouses in MLB history?
Some figures courtesy of Paul Hembekides of ESPN Stats & Info:
You can see why the Red Sox needed Martinez. Not only were their designated hitters terrible, but the middle of the lineup also was feeble, and the Red Sox finished last in the AL in home runs. With Martinez now the beast to build around, the Red Sox are second in the AL in home runs, and their runs per game has climbed accordingly:
2018: 5.46 (on pace for 885 runs)
2017: 4.85 (785 runs)
The Red Sox have scored 661 runs through 121 games; at last year's rate, they would have scored 587 runs through 121 games. That's 74 more runs -- but Martinez isn't responsible for all 74 of those runs. We can estimate that Martinez has created about 114 runs so far; last year, Red Sox DHs created about 97 runs (or 72 over 121 games). Martinez has created roughly 42 more runs so far than last year's DHs. (For this quick and dirty little study, we'll ignore that Martinez has started 44 games in the outfield. We're just trying to estimate how much of the offensive improvement should go to Martinez.)
Others deserve credit as well, most notably 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's already created more runs than he did in all of 2017 -- about 124 to 105, via the numbers at Baseball-Reference.com. Prorated over the same number of plate appearances, Betts has been an incredible 55 runs better.
Indeed, Betts' improvement has arguably provided a bigger impact than the addition of Martinez.
Of course, that ignores the trickle-down effect of adding Martinez. Maybe he provides lineup protection. Maybe he makes the other hitters better merely by his presence. Those are speculative ideas, however. One of the earliest and longest-running sabermetrics issues is the question of lineup protection, but comprehensive study after comprehensive study has failed to prove that such a thing exists. The Red Sox are better at the plate because Martinez is better than the players they had last year at his positions and because Betts is better than Betts.
All this ignores the pitching and defense. The Red Sox have allowed 3.69 runs per game compared to 4.12 in 2017 (52 runs better over 121 games). Even adjusting for the slight leaguewide downturn in offense, the pitching and defense have been better than last season.
Martinez has been incredible. He's been worth 5.6 WAR so far, on pace for a 7.5-WAR season. That's an MVP-level season, and his Triple Crown push puts him right up there with Jose Ramirez, Betts, Francisco Lindor and Mike Trout in the MVP race. If you dig deeper, you can find some added value from Martinez as well -- for example, he leads the majors in win probability added. He's hit .368 with runners in scoring position. He's hit .326/.442/.651 in "late and close" situations and .360/.462/.748 when the game is tied. Really, he's mashed in every situation.
The Red Sox may finish 22 games better. Martinez may be worth seven to 10 of those wins. And he may not even be the most valuable player on his own team.