FORT MYERS, Fla. -- Cuban outfielder Rusney Castillo will be out indefinitely after an MRI showed a strained left oblique muscle, Boston Red Sox manager John Farrell said Wednesday, an injury he sustained on his last at-bat in Tuesday night's exhibition against Boston College.

Editor's Picks Castillo injury clears the way for Betts in CF

"He's going to be down for some time," said Farrell, who indicated the imaging showed the strain was located in an internal layer of the muscle. "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."

Farrell would not say whether he believed the injury will impact Castillo's readiness for the regular-season opener in Philadelphia on April 6.

"Too early to tell," he said.

But Farrell acknowledged an oblique injury is typically approached with caution.

"There's caution to the progression, there's no doubt," he said. "[Pain] tolerance, for one. How aggressive you become to build that back. We just want to be careful of a setback, because everything is rotational in this game."

The injury places on hold what the Sox had said was a competition for the team's starting center-field job between Castillo, who last August signed a six-year, $72.5 million deal, and 22-year-old Mookie Betts, who had never played outfield before last season. Castillo insisted Wednesday that the injury will not prevent him from being ready for the start of the regular season.

Rusney Castillo is out indefinitely after straining his left oblique in his last at-bat Tuesday. AP Photo

"No, no, I don't think it impacts me in a negative way," he said through translator Adrian Lorenzo. "We're doing everything we can to recuperate as soon as possible, and we'll see how it goes."

But Castillo admitted he has never had a similar injury, and given the limited number of at-bats he has had since signing with Boston, even if he makes a quick recovery, the Sox may decide he needs more time.

After finishing last season with the Red Sox and batting .333 (12-for-36) in 10 games, Castillo played in the Arizona Fall League and in the Puerto Rican winter league, but a thumb injury limited him to 73 at-bats and 18 games split between the two leagues.

With Castillo sidelined, last season's center-field incumbent, Jackie Bradley Jr., will receive more playing time in center field in camp. Bradley was a Gold Glove finalist last season, but batted just .198 in 127 games for the Red Sox.

Castillo said he was injured while swinging at an inside pitch.

"It wasn't any sort of different kind of swing, or odd kind of swing; the pitch was a little in and I took a regular swing on it," he said. "I just felt something there, right in the oblique area."

Castillo said he felt better Wednesday than he had the day before.

"It's part of the process," he said. "It's frustrating. I don't know how much time I'll be out with it, but it's part of the process.

"Without going into real specifics, there's an established treatment plan over however many of days, and hopefully I'll get back out there as quickly as I can."