"[Castillo's] going to get reps in probably all three [outfield spots]," said manager John Farrell. "As he goes, there's going to be the earning of additional playing time."
Holt is likely to start on Opening Day for the first time in his career when the Red Sox face Indians righty ace Corey Kluber.
"It would be a cool experience," said Holt. "It was cool making the Opening Day roster last year and being a part of that. Getting to play on Opening Day would be pretty special. I know it's supposed to be pretty cold in Cleveland, but we'll just layer up and be ready to go."
The Red Sox signed Castillo to a seven-year, $72.5 million contract in August 2014, but the outfielder remains a bit of an enigma. Last season, he battled nagging injuries and struggled to be consistent at the plate.
In 329 career plate appearances, Castillo has a line of .262/.302/.379 with seven homers and 35 RBIs.
"He can play all three outfield positions. He's got power on the right-handed side of the plate," said Farrell. "Granted, the area that he continues to work at is just the offensive side of the game -- that's got to gain some consistency. And what's part of that is his own knowledge of who he is and what his swing is capable of and how that matches up with certain types of pitches."
The Red Sox have Jackie Bradley Jr. set to start in center and Mookie Betts as the mainstay in right.
Left field will be comprised of a committee led by Holt. Chris Young should see a lot of action in left against left-handed pitching.
Asked specifically if Holt is his main left fielder against righties, Farrell said, "At this point, unless he's on the infield somewhere, yeah."
In each of the past two seasons, Holt played every position for the Red Sox but pitcher and catcher. He became an All-Star in that roving role last year, hitting .280 with a .349 on-base percentage and .379 slugging over 454 at-bats. In 2014, Holt had 449 at-bats with nearly identical statistics.
"This is a guy, his value continues to grow," said Farrell. "His importance on our team continues to grow with each passing month just because of the versatility. Setting that aside, he's a good player. There's the utmost confidence to put him at any one of seven positions on the field, and that's a rarity. He's grown in his own confidence and he's done one hell of a job."
Holt started Tuesday's Grapefruit League game against the Twins at shortstop, and the Red Sox will continue to capitalize on his versatility when they can. But his most regular spot in the lineup will be as an outfielder, a position he virtually learned on the fly during the 2014 season.
"If that's where I end up the majority of the time, then I'm happy about it," Holt said. "We're all on the same team here, so regardless of who is playing where or when, we're going to root for each other. I got a little sense of maybe I would be out there over the past few days when I've been playing more out there. But I'm going to continue to do all my infield stuff and be ready to play wherever."