Cubs starting pitcher Cade Horton has been placed on the 15-day injured list with a strained forearm after leaving his second start due to discomfort. The team has recalled left-hander Riley Martin from Triple-A Iowa to cover Horton's position. Horton will undergo further tests and is expected to return to Chicago on Monday. The Cubs also recalled right-hander Ethan Roberts as the 27th man for the doubleheader.