Castillo, who was playing with Triple A Pawtucket, underwent an examination on Monday. His prognosis is "encouraging," the report says, but manager John Farrell is expected to provide more details on Tuesday.
Over three games with Pawtucket, Castillo went 5-for-12 with two RBI, a walk and a stolen base.
Over nine games this spring, Castillo went 9-for-29 with two home runs and five RBI.
In April, Castillo was optioned to the minors.
"The everyday at-bats are part of this, certainly, so is the depth and the overall roster we have here with us," Farrell said at the time. "The one thing he has yet to do since coming to the States is play every day for an extended period of time so that's all factored in."