Double-A Harrisburg placed Hassell on its 7-day injured list Wednesday with an unspecified injury, Bobby Blanco of MASNSports.com reports.
Hassell will join one of the Nationals' other top outfield prospects, James Wood (hamstring), on Harrisburg's injured list. Over 215 plate appearances at Double-A on the season, Hassell has hit .278 with three home runs and 11 stolen bases.