Swihart had been learning how to play left field since his demotion on April 15, and he'll stand in for Holt and play there against righties for the time being.
Swihart batted ninth and made his first Major League start in left during Friday's 4-2 loss. Swihart looked comfortable in his first start in left field, handling the four fly balls that were hit to him. At the plate, the switch-hitter went 0-for-3.
"He handled the chances he had cleanly," said Red Sox manager John Farrell. "He wasn't really tested in terms of any kind of throws. It's him getting acclimated to the position."
The one thing Swihart hadn't been able to practice at is playing the Green Monster, but he got that taken care of in early work before Friday's game.
"I played the outfield a lot when I was younger," Swihart said, "so I'm just trying to get those instincts to take over again. Just getting jumps on balls. I find myself backing up bases when I'm not even thinking about it, so the instincts are taking over."
Swihart has also been catching at Pawtucket, but he'll only throw on the gear during this stint with the Red Sox in pregame drills or warming up pitchers. Christian Vazquez and Ryan Hanigan are holding down the catching position.
For Holt, this is his second concussion. He also suffered one in 2014, forcing him to miss the final month of that season.
The Red Sox are hopeful the recovery time will be less this time. As with any concussion, the club will take every precaution and go through proper protocol before bringing Holt back.
"The only thing I can say is that Brock felt symptoms that are similar to the end of 2014, and in consultation with Dr. Collins in Pittsburgh, as has been done yesterday, we feel like this is clearly on the mild end of the range. But still, these take on an individual case by case in how they resolve and what's the next step to come back," said Farrell.
Holt is just 2-for-24 since making the diving play against Oakland that might have caused the concussion.
Swihart broke camp as the Red Sox's starting catcher, but he was optioned to Triple-A Pawtucket on April 15.
In 29 games for Pawtucket, Swihart hit .243 with one homer, eight RBIs and a .655 OPS. Swihart has started 11 games in left field and was errorless in 23 chances.
"Anything I can do to help this team win, that's what I want to do," said Swihart. "They're hot right now, having a good time, so I just want to enjoy that with them."