Brock Holt has been getting most of the starts in right field against righties of late. When the Red Sox recalled Castillo on May 22, they were hoping he could provide a spark to the offense.
But Castillo hit .230 with a homer and six RBIs in 74 at-bats.
"With Rusney, we just need to get him back and playing every day," said Red Sox manager John Farrell. "He had been kind of relegated to a bench role here, which, in fairness to him, was difficult to get into any kind rhythm and gain some confidence."
To fill Castillo's spot on the roster, the Red Sox activated catcher Erik Kratz after claiming him off waivers from the Royals over the weekend. Kratz will provide caching depth while Blake Swihart recovers from a left foot injury.
After signing Castillo to a seven-year, $72.5 million contract last August, the Red Sox still aren't sure what kind of player they have.
Castillo, who hails from Cuba, didn't play organized baseball for over a year before signing with Boston.
Castillo fared well in his initial stint with the Red Sox last September, hitting .333 with two homers and six RBIs over 10 games.
"He got pitched to differently in the time he was here this year than when he first came to us last year and what we saw in Spring Training," said Farrell. "He recognizes when there are pitches in certain parts of the zone that give him a little bit more difficulty than others as you're being pitched to.
"He needs to get back in the rhythm of the game more than anything," said Farrell. "The year missed, sure, that's a void. We can't go back in time. To say that there's development or specific focus on development, to me, it's more a matter of how you adjust to the way you're being pitched. The speed of the game and the strength of the game is better here than he's experienced elsewhere. So all of that goes into the reason to play him every day [in the Minors], so he can contribute here."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.