Mella reported to Triple-A Indianapolis on Tuesday after recently signing a minor-league contract with the Pirates, Jake Crouse of MLB.com reports.
The Diamondbacks released Mella on July 28.
Mella was designated for assignment by the Diamondbacks on Friday.
Mella had his contract selected by the Diamondbacks on Tuesday.
The Diamondbacks outrighted Mella to Triple-A Reno on Monday, Zach Buchanan of The Athletic reports.
Mella was designated for assignment by Arizona on Friday.
Mella (2-0) tossed a scoreless sixth inning to become the pitcher of record during the Diamondbacks' 3-2 Wednesday night win over the Rangers.
Mella's contract was selected by the Diamondbacks on Tuesday.
Mella will be a part of the Diamondbacks' 60-man summer camp.
Mella joined the Diamondbacks as a non-roster invitee Tuesday.