"It was my idea," said Castellanos, who added that he was open to playing in the outfield if it makes the team better.
Castellanos, who broached the topic with Ausmus and general manager Al Avila on Friday, was "gung-ho" about the possibility of playing right field, Ausmus said.
It's not the first time the 25-year-old has experimented with new positions since the Tigers selected him 44th overall in the 2010 MLB Draft out of Archbishop McCarthy High School (Fla.).
"I'm used to it, man," he said. "It's been a defensive merry-go-round pretty much for me since I got drafted. Third [base], right [field], left [field], back to third. Now back to right. I don't care. At the end of the day, I just want to win."
At 19, not long after he was drafted, Castellanos began working on a transition to the outfield. A year later, he appeared as a right or left fielder in 75 games during the 2012 season, split between the Arizona Fall League and Double-A Erie.
In 2013, he spent the full season in left, playing 130 games for Triple-A Toledo and nine games for the Tigers. But in the four years since, he's been strictly a third baseman for Detroit.
This year, Castellanos leads third basemen in errors (16). Among qualified third basemen, he ranks last in FanGraphs' defensive runs saved metric (-12) and is tied for last in fielding percentage (.938).
Castellanos remembers the extra work he put in every day as a teenager when he first made the move to the outfield. He said the transition is different this time.
"I'm way more of an adult than I was at 19 years old," Castellanos said. "And I know how to go about my business better than I did when I was 19. It's a good opportunity, and I'm excited for it."
Before Saturday's game against Minnesota, Castellanos worked closely with third-base coach and outfield instructor Dave Clark, who hit grounders and fly balls to Castellanos in the outfield while also demonstrating some footwork techniques.
Ausmus reiterated that Castellanos, who started Saturday at third, is in the infant stages of shifting to outfield. It's possible Castellanos sees time in the outfield at some point in September, but Ausmus suggested the team could wait until Spring Training to make an evaluation.
As for the implications of prospects coming up behind Castellanos and what his future might be with the Tigers, Ausmus declined to project. Castellanos appeared content to let that question linger.
"Baseball's such a present game that the future is so unpredictable," he said.