Ryan Mattheus. (AP Photo/Laurence Kesterson) Ryan Mattheus. (AP Photo/Laurence Kesterson)

Reliever Ryan Mattheus has been optioned to Class AAA Syracuse to make room for right-hander Blake Treinen, who will take Gio Gonzalez’s place in the Nationals’ rotation and start Thursday in Pittsburgh.

Mattheus, 30, was called up as an extra arm for the bullpen after Gonzalez landed on the disabled list with an inflamed left shoulder on Sunday. But when the Nationals needed a spot for the replacement starter, there was no longer a need for eight relievers. Mattheus fired 2 2/3 scoreless innings and allowed only one hit during this recent stint in the majors.

Treinen, 25, has started once already for the Nationals. He dazzled against the Dodgers on May 6, allowing three unearned runs on seven hits over five innings. At Syracuse, Treinen has a 1.96 ERA over six starts.