Gio Gonzalez had his first throwing session since last week’s MRI. (Mitchell Layton/Getty Images)

Gio Gonzalez played catch Saturday afternoon for the first time since an MRI last weekend showed inflammation in his left shoulder.

Almost five hours before first pitch in Pittsburgh, Gonzalez headed to right field at PNC Park. With pitching coach Steve McCatty watching, Gonzalez tossed lightly with head athletic trainer Lee Kuntz. The session went as planned, and Gonzalez will continue his rehab as scheduled.

“He was fine,” Manager Matt Williams said. “It’s the early-stage process. He didn’t get out there very far, but he reported that he feels good. So he did his exercises, went out there and threw, came back in and did some more exercises. He’ll stretch it out a little bit more tomorrow and work back to getting back on the mound. But he feels good.”

The Nationals are unsure how long, exactly, Gonzalez will be sidelined during the first disabled list stint of his career. But they expect it will be for three or four turns through the rotation. His replacement is Blake Treinen, who is 0-2 with a 1.56 ERA in 17 1/3 big league innings.