The players are dealing with a "condition that prevents them from reporting to the field," Astros manager Dusty Baker said, according to Chandler Rome of the Houston Chronicle.
Neither player has been at camp since training resumed, notes Chandler, and there is currently no timetable for their respective returns.
Alvarez, 23, hit the ground running in his first taste of the majors in 2019. He hit .313/.412/.655 with 27 home runs and 78 RBIs in 87 games.
Urquidy, 25, also made his MLB debut last season. After posting a respectable 3.95 ERA through 41 regular-season innings, he shined in the postseason, allowing one earned run over 10 innings of work while striking out 12.