To make room for Igarashi, oft-injured pitcher Dustin McGowan has been transferred to the 60-day DL.
Igarashi (1-1) has four saves and a 1.29 ERA in 19 games this season in Las Vegas. The 32-year-old Chiba, Japan, native went 5-2 in 79 games over two seasons with the New York Mets (2010-2011).
Cooper is batting .298 in 42 games with Las Vegas. He has 12 doubles, a triple, six home runs and 34 RBI during that span.