The 25-year-old Dalbec started hours after Boston traded first baseman Mitch Moreland to San Diego. Dalbec (2 for 4) struck out in his first at-bat before taking Javy Guerra deep over Pesky's Pole in right field in the third inning.
''I knew I barreled it - I didn't really know where at first,'' Dalbec said. ''Then I kind of saw it right over the pole, so I was just hoping that it stayed fair.''
Dalbec didn't find out he was being called up until late Sunday morning and left the team's alternate training site in Pawtucket, Rhode Island, at 11 a.m. in a rental car to make the one-hour drive north to Fenway Park.
''It was sweating a lot,'' Dalbec said. ''I think about all the tough times, you know, playing minor leagues, college, all that stuff. Everything's kind of led up to this moment. The day overall was kind of a fire drill.''
Devers had a two-run blast and a solo shot. Xander Bogaerts hit a two-run homer and Kevin Plawecki added two RBIs for the Red Sox. Josh Osich (1-1) tossed 1 1/3 scoreless innings after starter Zack Godley allowed five runs on eight hits in 4 2/3 innings.
''Everybody we bring in seems to be doing the job,'' Boston manager Ron Roenicke said about his bullpen, which has recorded 11 consecutive scoreless innings. ''Guys are throwing the ball really well.''
Boston won its second straight game and claimed its first series victory since taking two of three against Toronto from Aug. 7-9.
Trea Turner had three hits to extend his hitting streak to a career-high 15 games, and Josh Harrison and Eric Thames each had solo homers for the Nationals. Austin Voth (0-4) allowed five runs on six hits in two innings.
''We're starting to swing the bats. We're scoring runs. We've just got to get our starting pitching figured out, and then we'll go from there,'' Washington manager Dave Martinez said. ''But the lineup is starting to swing the bat.''
Washington has dropped five of six. Last year's World Series champion has the second-worst record in the National League.
''We never panicked last year, even when we were 19-31 and all that stuff in May,'' Nats reliever Sean Doolittle said after recording two outs in the seventh. ''As rough as things are right now, guys aren't panicking.''
Bogaerts' homer bounced off a light tower on the Green Monster during Boston's three-run first. Devers homered in the second and Dalbec's blast an inning later made it 8-3.
Juan Soto hit an RBI single and Asdrubal Cabrera had a sacrifice fly in the third. Harrison homered in the fourth and Thames went deep in the fifth before Eaton's RBI triple made it a three-run game.
Devers added an insurance run with his homer in the eighth.
Turner singled on Godley's fourth pitch of the game to extend his streak.
DEAL OR NO DEAL
After trading Moreland for Padres prospects Hudson Potts and Jeisson Rosario, Boston chief baseball officer Chaim Bloom did not rule out making more deals before Monday's trade deadline.
''I don't know,'' Bloom said. ''We certainly expect to be very active in conversations between now and tomorrow afternoon.''
Brock Holt didn't have to travel far to join his new team. The former Red Sox utilityman still lives in the Boston area despite the team moving on from him last offseason. After signing with Washington on Saturday, Holt only had to drive 20 minutes to Fenway Park.
''It happened to work out that they were here, and it just made things that much easier,'' Holt said. ''I was excited about the opportunity, excited that I didn't really have to go anywhere.''
A video tribute honoring Holt played before the third inning. Holt, sitting in the stands near the Nationals' dugout, jumped on the field for a curtain call and jokingly tipped his cap and bowed to thank a nonexistent crowd.
Nationals: Eaton was slow to get up after running into the wall in right after misplaying Devers' home run ball. He stayed in the game.
Red Sox: DH J.D. Martinez left the game in the eighth after being hit by a pitch on his left hand. X-rays came back negative and he is day-to-day.
Nationals: RHP Eric Fedde (1-2, 3.57 ERA) will face the Phillies for a second consecutive start in the first of three games at Philadelphia on Monday.
Red Sox: RHP Colten Brewer (0-2, 4.57 ERA) is set to work as an opener Monday to begin a three-game series against Atlanta.
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