"Yeah, he went through a battery of tests with Dr. Collins in Pittsburgh over the last two days," said Farrell. "He's traveling back to Boston here tonight, and after a full workup with Micky there, I feel like there's a very detailed treatment plan in place for him. He'll begin some general conditioning tomorrow when he gets back to us.
"He's still dealing with some symptoms, minor as they might be, and in the coming days, baseball activity will start at that point. We're going to miss him for a little bit. He's going to need some time to get back to game speed for us. But we don't feel like this is a real long-term type of scenario with Brock."
This is the second concussion of Holt's career. His first was in the 2014 season, when he missed the final three weeks of the season. Holt believes he suffered his latest concussion on a diving play at second base against Oakland on May 9, but he thought it was just a neck injury and played through it for another week.
Blake Swihart is playing left field against righties in Holt's absence.
• Lefty Eduardo Rodriguez is "tentatively" slated to make another rehab start for Triple-A Pawtucket on Sunday. However, Farrell said there's a chance the Red Sox will activate him for a start for Boston during the early part of next week. Rodriguez has been out since Feb. 27 with a patella subluxation in his right knee. He pitched seven strong innings for Pawtucket on Tuesday, allowing one run and striking out seven.
Ian Browne has covered the Red Sox for MLB.com since 2002. Follow him on Twitter @IanMBrowne and Facebook. This story was not subject to the approval of Major League Baseball or its clubs.