During the 2018 season, in an American League hotly contested with superstar teams such as the Yankees and Athletics, the Boston Red Sox emerged as the best team in baseball, relying heavily on the superstar bats of Mookie Betts and J.D Martinez. In fact, both Mookie Betts and J.D Martinez contributed 50% of the offensive WAR (according to Fangraphs) during the 2018 season, and Mookie Betts ended up winning the 2018 MVP over Mike Trout, who had an outstanding season that year. Pitching from the Red Sox was mainly carried by Chris Sale, who made up 1/3 of the total Pitching WAR during the 2018 season. With such outstanding players playing at superstar levels, it was only a matter of time until the Red Sox would regress and be more normal. For example, although a great player, Mookie Betts could not sustain a 10 WAR the next season (no player in the MLB today has sustained a 10 WAR in two seasons) and J.D Martinez performed well below the 5.9 WAR during the 2019 season. Although there were improvements to some of the other player's performances such as Xander Bogarts and Rafael Devers, it was not to offset the large decreases and the Red Sox's offensive WAR dropped from 32.1 to 27.5. Admittedly, the offensive is very good, but analysts would have probably expected the offense of the Red Sox to regress slightly.
However, the more glaring issue of the Red Sox would have been the decrease in pitching performance. Their pitching WAR decreased from 18.3 WAR to 16.5 WAR. Now the decrease might not seem glaring, but factor in the fact that the Red Sox dropped 6 places in rankings (in terms of pitching WAR), and the decline in performance from the 2018 season to the 2019 season would mainly encompass the pitching rather than the offense. Of course, the biggest sign would have been Chris Sale's performance, who has allowed quite a lot more home runs than in the past.
2018 Season:
2019 Season:
Now that Mookie Betts has been traded to the Dodgers and the Red Sox has been accused of cheating, the Red Sox seems once again at the rebuilding phase. Although I don't think that they need to rebuild their offense, if they really build up their pitching, then we could see a return of the Red Sox to the playoffs and even the World Series