They also optioned right-hander Taylor Hill to the minor league club before Saturday's day-night doubleheader against the Chicago Cubs.
Treinen was recalled as the 26th man and was scheduled to make his fifth career start in Game 2.
This is Cedeno's third stint with the Nationals this season. The left-hander had made two appearances with the major league club before being recalled.
Hill threw 4 1-3 innings over two appearances with Washington, posting a 4.15 ERA.