Aristides Aquino drove in three runs for Cincinnati, which had dropped four in a row. Mike Minor (3-10) pitched seven sparkling innings for his second straight win.
Cavalli, the No. 22 selection in the 2020 amateur draft, was tagged for seven runs and six hits in 4 1/3 innings. The right-hander, who turned 24 on Aug. 14, was called up from the minors earlier in the day.
''I felt very comfy, at home,'' Cavalli said. ''I really thought I was going to be more nervous, and I liked how I felt. I liked my mental space up there. There was no panic. I felt very composed. But I have to execute more. It comes down to that. You have to execute pitches and I didn't do that tonight and I didn't put my team in a position to win the ballgame. I have to do better.''
For Washington, the worst team in the majors and less than a month removed from trading Juan Soto to San Diego as the latest step in a multiyear teardown, Cavalli's debut was one of the season's most anticipated moments. He was the Nationals' minor league pitcher of the year in 2021, and was 6-4 with a 3.71 ERA in 20 starts at Triple-A Rochester this season.
Instead, things went awry almost immediately.
The Reds scored twice in the first. Jonathan India was hit by a pitch, moved up on a passed ball and scored on Washington shortstop CJ Abrams' errant throw on Solano's infield single. Solano then scored on TJ Friedl's double to right.
Solano's double in the third lifted Cincinnati to a 4-0 lead.
''You can't really judge a kid's first outing because of the nerves,'' Washington manager Dave Martinez said. ''No matter what they tell you, the nerves are there. He wants to impress and he wants to show he belongs here. I thought his stuff was really good.''
Cavalli (0-1) pitched a 1-2-3 fourth, but loaded the bases with one out in the fifth. He was pulled after hitting his third batter of the night, and Erasmo Ramirez promptly yielded Aquino's three-run double to left-center. Cavalli threw 99 pitches on the night.
''Despite those last few runs that came in, he was competing the whole time he stayed in the game,'' Nationals catcher Keibert Ruiz said. ''Being his first big league start, he was doing what we expected and making his pitches and competing. I thought he could have done a little better if we were more in sync, but I thought he did very well.''
Cavalli's outing marked the 42nd consecutive game a Washington starter did not earn a victory, the longest streak in major league history.
Minor was charged with two runs and six hits in his longest outing of the season. He reduced his ERA to a season-low 6.10.
Luke Voit hit a solo drive in the sixth for Washington. It was his fourth homer in 17 games since arriving from San Diego as part of the Soto deal.
Reds: India (left leg discomfort) was back in the lineup after missing Thursday's game. He reached safely four times and extending his hitting streak to nine games, a career-high... INF Mike Moustakas (left calf) missed his third consecutive game. ... OF Nick Senzel (left hamstring) was out of the lineup.
Nationals: Washington activated 2B Luis Garcia from the 10-day injured list and released 3B Maikel Franco. Garcia missed 11 games with a left groin strain. Franco hit .229 with nine homers and 39 RBIs, but lost the starting third base job to Ildemaro Vargas and had just one at-bat in the Nationals' last nine games.
Luis Cessa (3-2, 5.67 ERA) makes his third start of the season and second this week for Cincinnati. Cessa allowed two runs in two innings Monday at Philadelphia. Paolo Espino (0-5, 4.31 ERA) pitches for Washington. He is 0-5 with a 5.34 ERA in 13 starts since joining the rotation June 12.
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