Bradley, who is MLB.com's No. 11 overall prospect, had climbed up the Minor League ranks quickly after being drafted and was a possible rotation option for this season. But right elbow soreness slowed his progress, and he has spent most of his season with Double-A Mobile, where he has a 3.31 ERA in 11 starts.
Bradley going to the AFL means it's unlikely he will receive a September callup to the Major Leagues when rosters expand on Monday.
The D-backs' top two position players going to the Rafters, Drury and O'Brien, are ranked No. 6 and 7 in the D-backs' system, respectively.
Drury has split this season between Class A Advanced Visalia and Mobile, succeeding with both teams. Drury has a combined 23 home runs this season and an .892 OPS.
O'Brien, whom the D-backs acquired from the Yankees in the Martin Prado trade, has also shown substantial power, hitting 34 home runs across three levels of the Minors this season.
The final top-20 prospect the D-backs are sending to the AFL is Sherfy, who ranks 17th.
Sherfy has struggled to keep his run total down out of the bullpen with Visalia and Mobile, but his strikeout numbers have been a plus. In his short Minor League career, he is averaging 13.1 strikeouts per nine innings.
The other three -- Burgos, Fleck and Marzilli -- have had mixed results this season. Burgos and Fleck have had success, both posting ERAs under 3.00 with Visalia and Mobile, respectively. But Marzilli has struggled in Double-A, hitting only .248 with a .696 OPS in 75 games.
The Rafters open their fall season on Oct. 7 against the Scottsdale Scorpions.
Adam Lichtenstein is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.