Jon Heyman of MLB Network first reported the deal.
According to Jeff Passan of ESPN, the team would also see outfield prospect Khalil Lee go from Kansas City to the New York Mets and Royals outfielder Franchy Cordero to Boston. Right-handed pitcher Josh Winckowski—who was recently acquired by the Mets in the Steven Matz deal—is headed to the Red Sox, per Heyman.
Joel Sherman of the New York Post reported the Mets will also be including a player to be named later in the deal.
Benintendi, 26, hit just .103 with an RBI and four runs in 14 games and 52 plate appearances in a lost 2020 season. He looked like one of the game's young and upcoming stars after hitting 20 homers with 20 stolen bases and 90 RBI in 2017. He followed that up by posting superb numbers (.290 with 16 dingers, 87 RBI, 21 stolen bases and 103 runs) in 2018.
He wasn't terrible in 2019, hitting .266 with 13 homers, 68 RBI, 10 steals and 72 runs. But it was something of a down year and a precursor to 2020's letdown.
That the Red Sox are willing to give up on him this offseason is a surprise, given the potential he flashed in the past and the fact that they were shopping him while his value was depressed—but Boston chose to move on.
One concern is that Benintendi's speed has diminished throughout his career, per Alex Speier of the Boston Globe:
"The big league average sprint speed is about 27.0 feet per second. When Benintendi reached the big leagues in 2016, Statcast measured his average sprint speed at 28.6 feet per second—a mark that ranked as elite, the 89th percentile in the majors. His speed has declined in each subsequent year, down to a below-average mark of 26.6 in 2020 (again, small sample size warnings are necessary)."
One National League evaluator called it a "huge concern." An American League scout added that "his value takes a hit with me because I do not see him as a center fielder, nor a particularly good left fielder."
Grant Brisbee of The Athletic noted that Benintendi" has been chasing extra bulk for a couple of years now, thinking that he needed to add more power," perhaps with the unintended consequence that it simply slowed him down. If that's true and his speed reduction isn't a physical red flag, it's possible Benintendi could return to the player he was earlier in his career.
The Royals are making that bet.