Glasnow will be evaluated on a daily basis, according to the Pirates. He left his start for Double-A Altoona on Wednesday before the fourth inning after tweaking his ankle sliding into second base in the bottom of the third.
Glasnow, 21, is 2-1 with a 1.40 ERA and 28 strikeouts in 25 2/3 innings this season for Altoona.
The Pirates revealed no additional information before Thursday's 7-2 win over the Reds about right-hander Nick Kingham, the club's No. 6 prospect who left his start Wednesday for Triple-A Indianapolis in the sixth inning.
The initial report on Kingham was that he left due to elbow "discomfort." Catcher Tony Sanchez went out to the mound and motioned for a team trainer, who met with Kingham and Sanchez before manager Dean Treanor called for a pitching change.
Kingham, 23, is 1-2 with a 4.31 ERA in six starts for Indianapolis.
Meanwhile, the Pirates will get another right-hander back on the mound Friday night.
Charlie Morton, who began the season on the disabled list following labrum surgery in September, is set to begin his Minor League rehab assignment for Altoona in Harrisburg, Pa.
Morton will throw 80-90 pitches for the Curve, Pirates manager Clint Hurdle said. The 31-year-old is in good health, and Hurdle said his body is starting to sync up with his mechanics, which had been the cause of his struggles this spring.