Hassell was placed on the 7-day injured list Thursday with an undisclosed injury, Colin Beazley of the Pittsburgh Post-Gazette reports.
Hassell was promoted to Triple-A Indianapolis from Double-A Altoona on Aug. 18, but he did not make an appearance prior to the injury designation. He had been with Double-A Altoona since Aug. 8, and he picked up four hits with two RBI and six walks across 24 plate appearances in a five-game stretch.