Thomas left Chicago's game against the Tampa Bay Devil Rays in the fifth inning, after pulling up lame with a right hamstring injury while running between first and second bases.
"He said he felt something pop in his right leg," White Sox manager Ozzie Guillen told the newspaper.
Thomas told the Tribune he would know more about his condition on Monday.
Valentin also left the game, in the seventh, complaining of "irritation" to his left hamstring, according to the Tribune. The shortstop missed Friday's game against the Rays because the muscle was bothering him.
"In the last at-bat, I ran all the way hard and when I stepped on the bag I felt a twist," Valentin told the newspaper. "[Trainer Herm Schneider] said there was a little irritation."
The White Sox face the New York Yankees on Tuesday in a three-game series, though Valentin told the Tribune that he didn't know if he would be able to play, saying he has had such injuries before and didn't want to chance exacerbating it.
"I would rather miss a couple games and not a month," Valentin told the Tribune. "The way we're playing right now, I can't afford to go up there and be a hero and have it get worse."