Through the first eight games, the 28-year-old Castillo had started just once, and the club decided it would serve him better to get regular playing time at Pawtucket.
"I look at it as an opportunity to play every day and get better," said Castillo. "I'm excited at the opportunity to start every day, which is what I've always wanted to do."
"We need to get him out and get him going," said Red Sox manager John Farrell. "We needed to get some at-bats to get him playing every day."
Boston, which is off until Friday night's series opener against Toronto, did not make a corresponding roster move after Wednesday's game.
There's a chance that Castillo's spot will be filled by catcher Christian Vazquez, who is on a Minor League rehab assignment and recovering quickly from Tommy John surgery on his right elbow. Farrell said the Red Sox won't need an outfielder to replace Castillo.
Before Thursday's game, third baseman Pablo Sandoval, who also lost his starting job late in Spring Training, was placed on the disabled list with a left shoulder strain.
As for Castillo, he has been an enigma since the Red Sox signed him to a seven-year, $72.5 million contract in August 2014. Part of that has been due to a lack of opportunities, but Castillo has also struggled at times to stay healthy, and he hasn't proven himself offensively.
In 91 career Major League games, Castillo has slashed .265/.304/.383 with seven homers and 35 RBIs.
Castillo was also optioned at the start of the 2015 season, and one other time last season.
"I'm always ready for whatever news that's being given to me," Castillo said. "I'm just going to keep working hard, and we'll see what happens."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.