"The area is just showing some tenderness and soreness," said interim manager Torey Lovullo. "We want to get ahead of it, and it's almost there. Unfortunately, it's changed the plans that we laid down for the outfielders, but we'll continue what we've been doing the last couple of days and see where Rusney is tomorrow. But he is improving, just not quite there."
After having a solid August, Castillo's bat has been quiet in September. In 23 games, he's hitting .225 with no homers, four RBIs and a .555 OPS.
The Red Sox also continue to be without third baseman Pablo Sandoval, who has pneumonia.
"He's being evaluated still by our doctors in [Boston] and still showing signs of the pneumonia," said Lovullo. "Improving, but it's to the point where to resume all baseball activities would be very challenging for him. Rather than have him forced into a moment where he needs to play, we're going to say it's unlikely he'll rejoin us this year."
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, follow him on Twitter @IanMBrowne and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.