Rusney Castillo will be out indefinitely after an MRI showed a strained left oblique muscle, according to Boston Red Sox manager John Farrell.

Castillo sustained the injury in his last at-bat during Tuesday night's exhibition against Boston College.

"He's going to be down for some time," Farrell said. "I don't have a time frame to give you or projected length, but he's going through some treatment to calm down the strain right now, and he'll do rotational exercises and rehab when he's ready for it."

It's too early to tell if the injury will keep Castillo from playing in Boston's season opener against the Philadelphia Phillies on April 6.