[64] Announcers for spring training games broadcast on NESN were Dave O'Brien and Tom Caron for play-by-play with Jerry Remy, Dennis Eckersley and Lenny DiNardo for color commentary.

After Tampa Bay took a 5–4 lead in the top of the 12th inning, Martinez hit a two-out double in the bottom of the 12th with runners at second and third, driving in both runs and giving Boston a 6–5 victory.

Boston completed the sweep with a 9–2 win in the third game of the series, as starter Nathan Eovaldi went seven innings while striking out seven and holding the Rays to three hits and one run.

After Boston scored two runs in the top of the 10th, Matt Andriese held the Orioles scoreless in the bottom of the inning, securing his first save of the season and the Red Sox' fifth consecutive win.

[77] The Red Sox won the first game played in the series, 4–2, with reliever Adam Ottavino collecting the win and Matt Barnes earning his first save of the season.

Hernández, J. D. Martinez, and Alex Verdugo all hit home runs while White Sox starter Lucas Giolito was chased from the game in the second inning without recording an out.

The Red Sox ended April with a record of 17–10, good for first place in the American League East, 3+1⁄2 games ahead of the second-place Toronto Blue Jays.

The Rangers also won the final game of the series, 5–3, scoring three runs (two earned) in the bottom of the eighth inning off of Red Sox reliever Adam Ottavino, who took the loss.

Starter Nick Pivetta held the Marlins to two runs on seven hits, but left after 4+2⁄3 innings—the win was awarded to reliever Hirokazu Sawamura, the fourth of six Boston pitchers in the game.

Christian Arroyo became the first Red Sox player to be ejected during the 2021 season, via a ninth-inning strike-zone disagreement with home plate umpire Jim Wolf.

The win put Boston into a tie with Tampa Bay atop the AL East at 43–28; the Rays later lost to the Mariners, giving the Red Sox sole possession of the divisional lead for the first time since May 22.

Tampa Bay won the second game of the series, 8–2, as Boston starter Garrett Richards only lasted 1+2⁄3 innings while allowing five runs (four earned) on three hits and four walks.

With two outs in the ninth inning, Rays designated hitter Manuel Margot singled, stole second and advanced to third on a throwing error by catcher Christian Vázquez.

The game included the MLB debut of Jarren Duran, the ejections of bench coach Will Venable and backup catcher Kevin Plawecki following a check-swing third-strike call that the Red Sox dugout took exception to, and a brief stoppage in play after left fielder Alex Verdugo was struck in the back by a baseball thrown by a fan.

In the bottom of the inning, Boston scored twice, taking advantage of four wild pitches by Yankees reliever Brooks Kriske; Hunter Renfroe drove in the winning run via a sacrifice fly.

Alex Verdugo's own two-run homer in the eighth inning put Boston back in the lead, allowing Matt Barnes to close out the game with his 22nd save of the year.

[93] The Red Sox entered August in second place in the American League East with a record of 63–43, having just lost the first two games of a three-game series against the division-leading Rays (63–42).

[99] Tampa Bay won the game, 8–5, with Boston starter Brad Peacock taking the loss in his Red Sox debut after allowing five runs on two hits in 2+1⁄3 innings.

[102] With a record of 75–59, and having just lost the first two games of a four-game series against Tampa Bay, the Red Sox were positioned as a wild card team, in third place in the AL East.

Starter Nathan Eovaldi held Cleveland to three runs on six hits in 6+1⁄3 innings; he was relieved by Adam Ottavino, who was credited with the win; Garrett Whitlock earned a save.

The Red Sox' win put them back in second place of the AL East at 80–62 (.563), one-half game ahead of the New York Yankees at 78–61 (.561), with both teams in position as postseason wild cards.

Entering the top of the 10th, the Boston offense immediately went to work, as Jack López advanced to third base and then scored on a passed ball to give the Red Sox the lead.

The win moved Boston back to second place in the AL East, slightly ahead of the Yankees and Blue Jays, with the three teams contesting for two wild card spots in the postseason.

Records at the end of October 2: Red Sox (91–70), Yankees (91–70), Mariners (90–71), Blue Jays (90–71) Boston completed a sweep of the Nationals with a 7–5 come-from-behind win.

[123] For the ALDS roster, Boston added J. D. Martinez, Martín Pérez, Chris Sale, and Danny Santana, while omitting Jonathan Araúz, Matt Barnes, Jarren Duran, and Connor Wong.

[129] Prior to the second game of the series, Boston added Matt Barnes to their active roster, replacing Garrett Richards, who was placed on the injured list with a strained left hamstring.

An outfield assist by Kevin Kiermaier cut down Alex Verdugo at third base to end a Red Sox threat in the bottom of the eighth.

For the ALCS roster, manager Alex Cora made two changes to Red Sox' bullpen; Darwinzon Hernández and Hirokazu Sawamura were added, while Matt Barnes and Austin Davis were removed.

The Astros took a 1–0 lead in the bottom of the first; with two outs, Alex Bregman singled then was driven in on a double by Yordan Alvarez that hit off the glove of Red Sox center fielder Kiké Hernández.

After Carlos Correa was hit by a pitch from Tanner Houck, Kyle Tucker grounded into a double play, scoring Alvarez and giving Houston a 2–0 lead.