Nishioka, though, took it in stride, saying he plans on working hard to get another chance with the Twins.
"It's definitely tough to swallow, but it's not like it's getting baseball taken away from me," Nishioka said through translator Ryo Shinkawa. "I came from Japan for a challenge at this in this country. So I'm just going to not give up and look for the opportunity to get back up here."
Asked what he was told by manager Ron Gardenhire to work on down in the Minors, Nishioka said, "To slow the game down and be back to my old self when I was playing in Japan."
"He needs to play," Gardenhire said. "And as I told him, he needs to slow the game down and get back to having fun. Everything he does right now looks like it's work instead of a game. Maybe this will allow him to relax a little bit and just play."
Nishioka was competing for a spot as a backup infielder, but fell behind players such as Jamey Carroll, Alexi Casilla and Brian Dozier on the depth chart, according to Twins general manager Terry Ryan.
"He's behind some people," Ryan said. "I don't think that's a secret. We've got Carroll, we've got Dozier, we've got Casilla, we've got a few others. It's just a matter of time for him. We're going to send him down and see if we can get him to relax. He'll play some second and some short down there. Some of it's about positioning, you know, transitioning to the game here and let him relax. So I thought I'd make this move sooner rather than later to let people get organized, and especially him."
Nishioka was a five-time All-Star in Japan and won a batting title in his last season there in 2010 while helping lead the Chiba Lotte Marines to the Japan Series title.
But the switch-hitter struggled last season with the Twins, hitting just .226 with five doubles in 68 games. He also made 12 errors in the field, including 10 at shortstop and two at second base.
Nishioka was bothered by injuries, as he suffered a broken left fibula when the Yankees' Nick Swisher slid into him while trying to break up a double play at second base on April 7 that forced him to miss 59 games. He also missed time late in the year with a strained right oblique, and he played in his last game of the season on Sept. 9 as a result.
But Ryan didn't want to let those injuries serve as an excuse for Nishioka's struggles this spring, as he's currently healthy.
"I don't want to blame anything on injuries anymore, because he's healthy and everything is fine," Ryan said. "But he's got some things to work on, and he knows it."
With Nishioka out of the picture, Ryan said there's a possibility the Twins could look outside the organization to find a utility infielder.
Carroll is expected to be the Opening Day shortstop, with Casilla at second base. But outside of those two, the Twins don't have anyone on the roster with big league experience at shortstop other than Trevor Plouffe, who was moved to the outfield.
One option could be Dozier -- the club's Minor League Player of the Year last year -- but he's yet to play above Double-A and is not on the 40-man roster. Ryan, however, didn't rule out Dozier making the club, but also added it wouldn't be in a utility role. In that scenario, Dozier would be the starting shortstop, with Carroll serving as a utility infielder.
"We've taken a lot of players from Double-A through the years," said Ryan, who mentioned Joe Mauer, Chuck Knoblauch and Scott Erickson as examples. "That's not going to scare us off. If he looks like he's mature enough, or if any of these guys are ready enough."
Other Twins optioned to Triple-A Rochester on Monday were reliever Carlos Gutierrez and outfielder Rene Tosoni. Those reassigned to Minor League camp were relievers Jason Bulger, Luis Perdomo, Daryl Thompson, Esmerling Vasquez, P.J. Walters; catchers Chris Herrmann and Dan Rohlfing; infielder Aaron Bates; and outfielder Wilkin Ramirez.
With the roster moves, the Twins now have 45 players in camp: 21 pitchers (five non-roster), six catchers (three non-roster), 11 infielders (five non-roster) and seven outfielders (two non-roster).