New York sent minor league second baseman Hector Rodriguez and righty Jose Acuna to Cincinnati to complete the deal.
The 31-year-old Naquin gives the NL East-leading Mets a veteran outfielder and left-handed bat with solid career numbers (.280/.332/.473 with 50 homers) against right-handers. He has played mostly right field this season in Cincinnati but has experience in all three outfield spots.
Naquin is hitting .246 with 12 doubles and 31 RBIs this season, and slugged his seventh home run earlier Thursday, a solo shot in a 7-6 loss to the Miami Marlins.
He spent his first five seasons in Cleveland before signing with Cincy as a free agent last year. He’s a free agent again after this season.
Diehl, 28, has seen limited action in the majors, including just five relief appearances and an 11.12 ERA this season. His last appearance for the Reds came May 7, and he has since been pitching for the team’s minor league affiliate in Louisville, compiling a 4.24 ERA with 30 strikeouts in 25 games.
Major League Baseball’s trade deadline is Tuesday.
Read More News: Mets add veteran OF Naquin ahead of deadline