Paco Rodriguez (elbow bone spur) and Pedro Baez (pectoral strain) also could be activated from the disabled list within the next week, Mattingly said.
League has been out the longest, beginning the season on the disabled list with a right shoulder impingement. His outing on Wednesday was the eighth appearance of his rehab assignment, during which he has allowed only one run in 8 2/3 innings. He allowed one hit, had one strikeout and a line-drive double play.
League is coming off a bounce-back 2014 season, when he posted a 2.57 ERA after a 5.30 ERA in 2013. He was expected to help provide a late-inning bridge to closer Kenley Jansen this year, which has become a focal point of concern for the club. The Dodgers have had six relievers on the disabled list this season.
Mattingly said outfielder Carl Crawford, out nearly two months with a strained oblique, will resume hitting this week, and the next step for him would be rehab games.
Also in the game for Oklahoma City, shortstop prospect Corey Seager homered twice.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.