The Pirates placed Horwitz (hamstring) on the 10-day injured list Thursday with a left hamstring strain.
Horwitz was hurt during Wednesday's win over the Mariners. There's no word yet on the severity of the strain, so more will be known about a timeline for a return soon. Horwitz will be eligible for activation in early July, but with the All-Star break looming, there's a good chance Horwitz is shelved until the second half even if his strain is mild.