The Pirates placed Horwitz on the 10-day injured list Thursday due to a left hamstring strain.
Horwitz suffered the injury during Wednesday's win over the Mariners. The Pirates haven't provided word on the severity of the strain, making it difficult to forecast a timeline for Horwitz's return. Horwitz will be eligible for activation in early July, but with the All-Star break looming, the Pirates could proceed cautiously with the first baseman and keep him on the shelf until the start of the second half, even if his strain is mild.