Casilla, who injured himself while legging out a double in the ninth inning of Wednesday's 7-2 win against the Rangers, was placed on the 15-day disabled list. Infielder Luke Hughes was recalled from Triple-A Rochester.
Additionally, to bolster their middle-infield depth, the Twins recalled infielder Matt Tolbert from Rochester and optioned left-hander Chuck James to Rochester to give the Twins 12 pitchers on their active roster.
"He's our second-hole hitter and was swinging great and playing great defense," Twins manager Ron Gardenhire said of Casilla. "He's doing all the things you'd like to see, and his head has been in the game. And then all of a sudden this happens. Our depth in the middle infield isn't the greatest, so you can't afford too many injuries like that. Some people will have to step up."
Casilla, who was hitting .259/.322/.369 with 15 stolen bases and a team-high 21 doubles in 96 games, said he's hopeful that he can return to action as soon as he's eligible to be activated from the DL on Aug. 12.
"It's pretty much the same as yesterday," Casilla said. "I kind of pulled it a little bit, so I'm going on the DL and going back to Minnesota to take care of it over there. I don't think I'll need a [rehab] assignment. I'll be ready in 15 days. ... I hope they don't send me on an assignment. I'll say no because it's too much time."
In the meantime, the Twins will have to rely on a combination of Tolbert, Hughes and Trevor Plouffe at second base with Casilla out of action.
Tolbert, who hit .181 in 55 games with the Twins before being optioned on July 18, started at second base against the Rangers on Thursday.
And shortstop Tsuyoshi Nishioka is expected to bat second in Casilla's absence, as Gardenhire said he's hopeful he can handle the bat the way Casilla did in the two-hole.
"I'm hoping Nishi handles it very well," Gardenhire said. "When we signed him, we were hoping he'd be at the top of the lineup. So maybe this will get his confidence going a little bit."
Texas heat forces Twins indoors
With the Texas heat reaching 100 degrees for the fourth straight game against the Rangers, the Twins decided to skip outdoor batting practice before Thursday's game, opting to hit indoors instead.
The decision was also made because the club is flying to Oakland after the game to get ready for the three-game series against the A's that begins Friday night.
"We're staying out of the sun," Gardenhire said. "We've done enough work in the sun this week, and it's been hot. So we decided to give them a back-off day to hit in the cage. It was hot yesterday, and the guys have been getting after it. So we decided to back off tonight with the late flight."