"He's a horse," Callaway said before the Mets took on the San Francisco Giants, according to Anthony DiComo of MLB.com. "But sometimes, you've got to take it out of their hands."
Alonso, who carried a .280 batting average into the All-Star break, is just 3-for-30 with 13 strikeouts since returning from Cleveland.
Callaway said his decision to sit Alonso for the first time since June 11 came after a discussion with Todd Frazier, who informed him how taxing the Home Run Derby could be. Frazier participated in the event for three straight years from 2014-16, winning it in 2015.
Alonso swatted 57 dingers in his Derby, including 23 in the final round to win it. The rookie played in the All-Star Game the next day.
"I don't feel worn down," Alonso said. "I gotta trust my coaches and the staff have my best interests - I know they have my best interests at heart. I hate off days, I want to be in there because I'm really competitive ... but I respect the decision."
Alonso has been one of the few bright spots in the Mets' disastrous campaign. Even with his recent struggles, the 24-year-old owns a slash line of .265/.358/.606 with 32 homers and 72 RBIs for the season.