"While he's improving, he's still going through some vision exercises," said Red Sox manager John Farrell. "He has a re-examination and a battery of tests scheduled for Thursday with Dr. Collins. I think that will be a pivotal day. A pivotal day in the sense of where he's at and the projection for him going forward.
"And until we get that information, I have no real date marked for when he can be on the field. He'll be on the field when he's ready, but Thursday is going to tell us a lot."
Holt last played for the Red Sox on Sept. 5.
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. This story was not subject to the approval of Major League Baseball or its clubs.