Arroyo was 7-4 with a 4.08 ERA in 14 starts for the D-backs in 2014, but has missed all of this season while recovering from Tommy John surgery. The 38-year-old right-hander has played 15 years in the Majors.
Toussaint, 19, was selected by the D-backs in the first round of the 2014 draft at No. 16 overall. He was 2-2 with a 3.69 ERA in seven starts for Single-A Kane County this season. He was Arizona's No. 4 prospect, according to MLB.com.
D-backs general manager Dave Stewart said it would have been tough for the club to fit Arroyo into their rotation with his return nearing.