Blake Treinen (Kelley L Cox-USA TODAY Sports)

Blake Treinen will start the second game of the Nationals-Cubs doubleheader in Chicago on Saturday, according to Manager Matt Williams. The Nationals needed a starter for the second game if they didn’t want to pitch a starter on short rest.

Gio Gonzalez is slated to start the first game at 1 p.m. Eastern time, and Treinen will start the second game at 7:15 Eastern time. Treinen is on schedule to start; he last pitched at Class AAA Syracuse on Sunday. The Nationals will take advantage of the 26th-man rule to call up an extra player for the doubleheader only.

Treinen has made four starts for the Nationals this season and pitched in relief four times. The right-hander has a 2.08 ERA in 30 1/3 innings, striking out 17 and walking 10. He has an 0-3 record.

>>> Bryce Harper’s rehab assignment will move to Class AA Harrisburg on Thursday, Williams said. Harper is resting his surgically repaired left thumb on Wednesday and will resume playing with Harrisburg, which is in Akron, Ohio, for a four-game series.

“He could play all four depending on how he goes, how his legs are feeling, how his thumb feels,” Williams said. “We’ll evaluate day to day with him.”

Harper has already played two rehab games with Class A Potomac, starting in left field and in center field, and has gone 3 for 4 with a home run and four RBI. The Nationals have targeted July 1 as an approximate return date for Harper.

With Harper joining Harrisburg, he will be on the same team as his brother, Bryan, for the first time since they played for the College of Southern Nevada in 2010.

>>> Wilson Ramos is traveling to Chicago to meet the team Wednesday and is expected to be activated when he is eligible to come off the disabled list on Thursday, Williams said. Ramos caught nine innings on Tuesday for Class A Potomac and had no issues with his hamstring. He has been out since June 11.