Holt had concussions in 2014 and '16, and his vertigo symptoms could be related.
"I can't say that one is exclusive of the other. I am not trained in that area. We're going based on the feedback that our medical staff, the doctors here, Dr. Collins in Pittsburgh have given," said Red Sox manager John Farrell. "As we've seen some improved conditions, then there has been a step back, so that's where a lot of the frustration is that Brock has been dealing with directly. To say one is a cause of the other, I don't know that."
How long Holt is kept out of games all depends on his symptoms.
"Some additional maintenance work dealing with the symptoms that are there," said Farrell. "We're going to hold him out of playing for the foreseeable future, for the time being, until he gets past these symptoms completely. He'll remain with us. He'll keep working out. I think until he puts a little distance behind him with some of the things he's been dealing with, that is when we'll get him back in game situations. I don't have a time frame on that right now."
Holt was placed on the disabled list April 21. Farrell remains confident he will play again this season.
"And that's because of the confidence that Dr. Collins has in taking this approach right now," Farrell said. "He's going to need a little bit of time to get away from game activity. We'll still maintain the work on the field with all the drills, the BP, the defensive work, that will still remain, but we will just hold him out of competition."
• A day after Dustin Pedroia was removed from the game for precautionary reasons due to pain in his surgically-repaired left knee, Farrell held him out of the starting lineup Friday for the opener of a three-game series against the Mariners.
"I'll be honest, he wanted to get in the lineup, but more precautionary, I am holding him out," Farrell said. "Not going to take any further risks, particularly with all the work he's gone through on that knee. He's available, but staying away from him."
• Third baseman Pablo Sandoval, who is coming back from a right knee sprain, was slated to play nine innings at Triple-A Pawtucket on Friday and Saturday, and the team will make an evaluation after that if he needs more time on his rehab assignment.
• Red Sox utility infielder Marco Hernandez underwent a successful left shoulder open stabilization procedure Friday, and the club expects he will be ready for the 2018 season.