The Pirates placed Mattson on the 15-day injured list Tuesday with hip inflammation, retroactive to Sunday.
Mattson gave up three earned runs in two innings during his first appearance back in the majors since mid-August on Saturday and appears to have picked up an injury in the process. He'll be eligible to return during the final week of the regular season. In the meantime, the Pirates will bring up Brandon Eisert to fill the void in their bullpen.