Discipline at the plate topped the to-do list Rosario took with him to Triple-A.
"You can't make a guy walk more, but you can try to get him to change his approach," manager Paul Molitor said.
Rosario hit .267 with 13 home runs and led the majors with 15 triples as a rookie last year, but he was demoted with a .227 on-base percentage and 31 strikeouts in 115 at-bats this year. Oswaldo Arcia has taken over as the primary left fielder in place of Rosario, who has also committed his share of base-running and fielding mistakes.
"We have a lot of people who haven't performed as we hoped," Molitor said.
Center fielder Byron Buxton and starting pitcher Jose Berrios, widely viewed as the organization's top two prospects, have been sent back to Rochester after rocky stretches with the Twins. General manager Terry Ryan said the setbacks of 2016 have not forced him to reconsider his faith in the franchise's future. The young players have been put on notice, though, that growing pains simply won't be accepted without ownership of the work required to improve.
"We can't have any comfort in that clubhouse," Ryan said, "and I mean none."
Outfielder Robbie Grossman had his contract selected from Rochester to replace Rosario on the roster Thursday before the game against Toronto. The 26-year-old Grossman signed with Minnesota this week after opting out of his minor league deal with Cleveland. The switch hitter played one game for Rochester after spending the first two months of the International League season with Columbus.
Grossman has a .240 career batting average in 190 major league games, all with Houston over the past three years.
Rosario was taken aback a bit by the news, "which isn't a bad thing," Molitor said. The 24-year-old fourth-round draft pick in 2010 has a diverse set of skills but is hardly guaranteed a hold on a role as a regular.
With Buxton, Max Kepler and Adam Walker in Triple-A behind Arcia, Danny Santana and Miguel Sano in the Twins outfield, Rosario could find himself passed by if he doesn't display some improvement with Rochester.
"He's going to have to get to work fast," Molitor said.
Molitor, a Hall of Fame player with 3,319 career hits, walked only 19 times in 125 games as a rookie for Milwaukee in 1978. Molitor brought up that statistic himself, admittedly with reluctance, as an example of how discipline at the plate can be developed.
"In general, yes, people can improve," Molitor said.
But can the Twins, at least in this almost-already-lost season? They are currently tied with the Atlanta Braves for the worst record in the majors (10-29).
"We better hurry up and get going," Ryan said. "This isn't pretty at all."