Cashner will report to the Red Sox on Sunday and start for his new team on Tuesday against the Blue Jays at Fenway Park.
"He is excited to be here and excited to pitch," president Dave Dombrowski told reporters, according to Ian Browne of MLB.com.
The 32-year-old appeared to be happy in Baltimore, and in May he considered not reporting to a new team if a trade happened. But now it seems Cashner is happy with how everything turned out.
"It's definitely exciting," he told MASN's Roch Kubatko.
"David Ortiz has been one of my favorite players growing up. ... They're still the champions from last year, so it will be fun to help them get to where they want to go."
Cashner is in the midst of a solid 2019 season, his 10th in the majors. He owns a 3.83 ERA, 4.25 FIP, 1.19 WHIP, and has struck out 66 batters while walking 29 across 17 starts for Baltimore.
His addition addresses an immediate need for the Red Sox, who lack pitching depth. Cashner will likely become their No. 5 starter immediately, filling the hole left by Nathan Eovaldi, who will move into a relief role after returning from the injured list later this month.
"(Cashner also) helps our bullpen also because we think it will give us more length," Dombrowski added, according to Chris Mason of the Eagle-Tribune.
Cashner is in the final season of a two-year, $16-million contract and is earning $8 million this year. His deal contains a $10-million option for 2020 that becomes guaranteed if he throws 340 innings in the 2018 and 2019 seasons combined, and if he reaches 360 innings it becomes a player option. He's thrown 249 1/3 innings over the life of his contract to date.
The Orioles added cash into the trade to escape both the option and multiple performance incentives in Cashner's contract, Ken Rosenthal of The Athletic reports.
Baltimore sent approximately $1.8 million in cash to Boston - enough to cover over half of his remaining 2019 salary - and agreed to cover "most" of his bonuses should he reach them, a source told Sean McAdam of Boston Sports Journal.
The Red Sox signed Romero and Prado out of Venezuela last year, according to Jon Meoli of the Baltimore Sun. They are both 17 years old and have yet to play for a Red Sox affiliate in the United States.
Romero, an infielder who primarily plays third base, is slashing .264/.336/.364 with two homers, 13 RBIs, and two steals over 123 plate appearances with DSL Red Sox 2 this year. Prado, an outfielder, owns a .303/.400/.418 line across 147 plate appearances with the same team.