He is on the transfer list and Paolo Di Canio was hoping for a deal before the summer transfer window closes, but that won't be happening.
The full-back suffered the injury in the Black Cats' under 21s win over Bolton on Monday and was substituted at half-time.
Now reports say, after scans, Bardsley has broken his foot and he is set to be on the sidelines until the end of October at the earliest.
It's another blow to the player, who has already been told he does not figure in boss Paolo Di Canio's long-term plans.
The injury may mean he will have to wait until January until he can secure a switch away.