"OK, you're starting at shortstop tomorrow," Gardenhire replied.
And just like that, the 24-year-old infielder made his big league debut and batted ninth as the Tigers opened a three-game series against the Twins with a 4-2 win at Target Field on Tuesday.
In the top of the eighth, Castro poked a leadoff single through the right side of the infield for his first career big league hit. Later in the inning, he collected his first stolen base when he swiped second after the out call was overturned by replay. Castro came around to score on a Nicholas Castellanos single.
"It's gotta be really exciting, he's waited a long time for this moment," Gardenhire said. "As a manager, you really appreciate those things, giving a guy an opportunity and watching him do that."
Castro was so nervous Monday night that he could hardly sleep. He has played just 23 games at shortstop in the Minors this season and got most of his work at third base over the past two months.
"It's my first time in a starting lineup for MLB, so I'm real nervous right now," Castro said. "When I take the first ground ball or take the first pitch in my first AB, everything is going to change."
He won't be taking in the experience on his own. When Castro received the call from the Tigers and was told that he would be promoted to add infield depth over the final week of the season, he was sitting at home in Caracas, Venezuela. He immediately booked a flight to join the club, and his father caught the same flight; he wasn't going to miss his son's first Major League appearance. On Tuesday, he was in the stands at Target Field.
In 351 Minor League at-bats across Double-A and Triple-A this season, Castro hit .265 with two home runs and 29 RBIs while playing five different positions. He came on as a pinch-runner on Sunday, but had yet to start a game until Tuesday.
Jarrid Denney is a reporter for MLB.com based in Minneapolis.