Gio Gonzalez during a May 17 start, the day before landing on the DL. (AP Photo/Alex Brandon)

Gio Gonzalez is expected to rejoin the Nationals starting rotation early next week following two rehab starts at Class A Potomac. And in the meantime, the Nationals optioned Gonzalez’s rotation fill-in, right-handed starter Blake Treinen, to Class AAA Syracuse, and recalled left-handed reliever Xavier Cedeno from Syracuse. The Nationals will employ an eight-man bullpen until Gonzalez returns.

Gonzalez has been on the disabled list since May 18 with left shoulder inflammation. He threw four innings and 79 pitches for Potomac on Thursday, his second rehab start. Barring any issues with his in-between-starts bullpen session, Manager Matt Williams said Gonzalez will return next week.

“He felt good [Thursday] night,” Williams said. “No pain, no discomfort in the shoulder. He got to 79 pitches. He feels good about it. Given the fact that he’s slated to come back, we decided to make the move and get Xavier here. We’ll make a decision on Gio depending on how these three games go and where we slot him.”

Gonzalez’s command improved Thursday, though he still walked four. Williams said he was fine with that because Gonzalez was making a concerted effort to work on his breaking balls. “Velocity was good,” Williams said. “Health was good. He should be good to go.”

Gonzalez could start for the Nationals on Tuesday against the Houston Astros on regular rest, but that would push Tanner Roark back to Wednesday and make his start on seven days rest. If Roark starts Tuesday and Gonzalez on Wednesday, each would be starting with an extra day of rest.

Cedeno joins the Nationals for, at least, the weekend and could see action in St. Louis. The Cardinals are starting four left-handed batters on Friday night against Jordan Zimmermann, and have another left-handed batter on the bench.

Cedeno is in his second stint with the Nationals, having appeared in only one game — a scoreless 1 1/3 innings on April 21 against the Angels. In 14 2/3 innings at Syracuse, Cedeno has a 1.23 ERA and has a stellar 6.7 strikeout-to-walk ratio. He arrived in the clubhouse in St. Louis in the late afternoon, his teammates already there.

Treinen returns to Syracuse to continue this development as a starter. He made three starts in Gonzalez’s place and allowed two runs in each of them. During this major league stint, his second of the season, Treinen posted a 2.89 ERA over 18 2/3 innings and impressed officials with his high-octane sinker.

“I think Blake did a really nice job,” Williams said. “He pitched well, competed and all of that. But of course we want Gio back. He’s won a lot of games in the big leagues and we want him to be part of the rotation. So we’re glad to have him back in it, healthy and feeling good. That’s a good sign.”