Archie Bradley (5-1) allowed a run in the top of the ninth but got the win.
Josh Bell tripled and drove in two runs for the Nationals, who have lost five straight and 11 of 14.
Joe Ross pitched five scoreless innings, allowing three hits with four strikeouts and two walks, in his return to the Washington rotation after being sidelined since July 4 due to right elbow inflammation.
Spencer Howard breezed through three scoreless innings of one-hit ball before running into trouble in the fourth. Bell atoned for his first-inning blunder, when his lackadaisical play loping to the bag at first base allowed Realmuto to reach on an infield single, by tripling home the first run.
Andrew McCutchen rips walk-off, 3-run HR as Phils stun Nats
Field Level Media
27 Jul 2021, 13:19 GMT+10
Andrew McCutchen hit a walk-off, three-run home run off closer Brad Hand and the host Philadelphia Phillies rallied for a 6-5 victory over the Washington Nationals on Monday.
Rhys Hoskins also hit a three-run homer for the Phillies, who moved over .500 at 50-49 to crawl within 3 1/2 games of the first-place New York Mets in the National League East.
Didi Gregorius, Jean Segura and McCutchen had two hits apiece. Bryce Harper walked three times and scored a pair of runs.
Phillies starter Spencer Howard lasted only 3 2/3 innings and allowed six hits and four runs to go along with two strikeouts and no walks. Howard reportedly left with a blister on his right middle finger.
Nationals activate RHP Joe Ross from injured list
Field Level Media
27 Jul 2021, 06:19 GMT+10
The Washington Nationals activated right-hander Joe Ross from the 10-day injured list to start Monday night s road game against the Philadelphia Phillies.
In a corresponding transaction, Washington optioned outfielder Yadiel Hernandez to Triple-A Rochester.
Ross, 28, was 5-8 with a 4.02 ERA in 16 starts before landing on the IL with right elbow inflammation on July 8.
Over his past six starts dating back to June 5, Ross struck out 44 batters in 39 innings and posted a 2.31 ERA.
Hernandez, 33, is batting .253 with two homers and six RBIs in 57 games with Washington this season.
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Nationals vs. Phillies Prediction
The slumping Nationals will look to get back on the right track when they open a new series against NL East rivals the Phillies at 7:05 p.m. ET. With Joe Ross set to oppose Spencer Howard, what’s the top play when it comes this matchup?
Game Snapshot
7:05 p.m. ET, Monday, July 26, 2021
Citizens Bank Park, Philadelphia, PA
Washington Nationals DFS SPIN
Ryan Zimmermanbelted a three-run homer and plated four runs on Sunday in the Nationals loss to the Orioles. Zimmerman put the Nationals on the board with an RBI ground out in the opening frame and also walloped a go-ahead three-run homer in the sixth inning off lefty John Means. The 36-year-old veteran slugger has gotten off to a slow start at the plate, collecting just three hits in 21 at-bats over seven games since the All-Star break.