Walters, 27, had his contract purchased from Triple-A Rochester on Wednesday after left-hander Matt Maloney was designated for assignment.
Walters made six starts with the Red Wings, posting a 2.70 ERA with 25 strikeouts and six walks. Saturday will mark his fifth career big league start, as he has a career 7.24 ERA in 20 games with the Cardinals and Blue Jays.
He said he heard of the news late Wednesday, when Triple-A manager Gene Glynn called him to tell him he'd be joining the Twins rotation with left-hander Francisco Liriano headed to the bullpen.
"Obviously, I was excited," Walters said. "He just kind of told me I was going up and pitching on Saturday, so make the most out of the opportunity."
Walters, who said he throws a fastball, cutter, curveball and changeup, said he hopes his big league experience will serve him well, but that he always gets butterflies before every start.
"There's nerves, even in Triple-A, there's nerves," Walters said. "You care about what you're doing, you try to win. It doesn't matter where you're pitching."