Bradford, 27, appeared in just five games at the big league level after receiving a promotion late last month.
On Sunday, he tossed two innings and allowed a two-run home run to Charlie Blackmon.
Robles, 26, was just 4-1 with a 6.23 ERA, 6.23 FIP, 1.569 WHIP and 9.6 K/9 in 21 games with New York earlier this season before being sent down at the end of May.
However, Robles has allowed zero runs in his last five outings (6 2/3 innings) and seven of his last eight appearances after getting off to a rocky start in pitcher-friendly Las Vegas this season.
For the 51s this season, he is 0-1 with a 5.79 ERA in 23 1/3 innings tossed.