Rosario sustained the injury sliding into first base head first, trying unsuccessfully to beat out an infield hit to end the Twins' half of the 10th during Saturday night's 3-2 loss in 12 innings. He had to come out of the game.
In the fourth inning, Rosario hit his 10th homer of the season.
Twins manager Paul Molitor said that X-rays on Saturday night were inconclusive, and more tests will be conducted on Monday when the Twins return to Minneapolis to confirm the diagnosis. They open their final six-game homestand of the season at Target Field on Tuesday night against the Tigers.
"Rosario is probably not available [Sunday]," Molitor said. "We're going to get him checked out again [Monday] and make sure he doesn't have a fracture in that thumb. The X-ray he had here was a little inconclusive, given the swelling. That's why they want to have it re-done or get a little bit of more clarity tomorrow morning."
Molitor said he wasn't sure if doctors might opt for more extensive tests.
"They might go to a bone scan," he said. "It'll probably start with a generic exam and X-ray and go from there, go through the gamut of what you need to do to get the right answer."
Rosario is a .269 hitter with the 10 homers and 32 RBIs in 92 games.
Barry M. Bloom is a national reporter for MLB.com and writes an MLBlog, Boomskie on Baseball. Follow @boomskie on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.