Scratched from the Angels' lineup moments before the start of Saturday night's 7-1 loss to the Los Angeles Dodgers, their final game before the All-Star break, Trout has missed the past four games with back spasms, and on Sunday, he elected not to participate in Tuesday's All-Star Game in Los Angeles.
"It's just one of the things that's got to get right before I start swinging," Trout said. "It is frustrating for sure. I can't really pinpoint what caused it. It just started to bother me."
Trout was in the Angels' posted pregame lineup as the center fielder but was removed about three minutes before the first pitch. Trout said afterward that his back felt wrong while swinging in the cage before the game.
Originally scheduled to start in the outfield Tuesday for the American League, Trout is batting .270 with 24 homers and 51 RBIs in 79 games this season, although he has just one multihit game and six total RBIs in the past three weeks.
His transaction was one of several roster moves from the Angels, who claimed infielder Phil Gosselin off waivers from the Atlanta Braves and outrighted outfielder Monte Harrison to Triple-A Salt Lake after he cleared waivers.
Later in the day, the Angels announced on social media that Trout was named captain for Team USA in next year's World Baseball Classic.
"It means a lot. I missed an opportunity the first time and I knew this was one I couldn't miss," Trout said. "I'll be able to ramp it up a little bit quicker. I'm looking forward to playing for our country."
The Angels, who return to action on Friday against the Braves, entered the break at 39-53, 20½ games behind the Houston Astros in the AL West.
The Associated Press contributed to this report.