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.
D-backs manager Chip Hale said it was sad to see Arroyo's veteran leadership in the clubhouse go, but he understood it's a move that fits with the club's mindset of younger pitching.
"It's tough to lose those guys, but you have to do what's best for the organization, and we're trying to put ourselves in a position to be more competitive," Hale said. "That's just the way things go."
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. The move also allowed Arizona to get rid of Arroyo's salary, which is $9.5 million for this season.
"In order to move the contract, Touki had to become a part of it," Stewart said. "For what we're trying to do, the money is definitely important. We've said we're going to try to do what we can to promote our young pitching, but also have some savings so we can have the opportunity to do things at a later date."
Gosselin is currently on the 15-day disabled list with a left thumb fracture. The infielder has batted .325 with a pair of RBIs in 20 games for the Braves this season.
Stewart said the Braves told him that Gosselin is still four weeks away from returning from injury.