The Rangers announced Sunday that Kirkman has a recurrence of a cutaneous lymphoma located in the area of his right triceps. He will begin treatment soon and will still be able to participate in baseball activities.
The club said that a complete recovery is anticipated, but there is no timetable for Kirkman's return. The Rangers requested that everyone respect Kirkman and his family's privacy during this time.
Kirkman is 0-2, with an 8.18 ERA in 25 relief appearances. He did not pitch in the Rangers' 18-inning loss to Toronto on Saturday. McClennan, 29, is 1-0 with a 2.57 ERA in six relief appearances for Triple-A Round Rock.
The recurrence of the skin cancer of about 2 millimeters from the original location that was discovered during a routine exam in May. Kirkman was originally diagnosed with the cancer in Jan. 2012, and went through radiation treatments. The cancer was believed to be in remission. Kirkman will begin radiation treatment shortly.
"This had to be weighing on his mind since this was discovered in May," Rangers assistant general manager Thad Levine said.