The 31-year-old has lost his way since joining the Villans from Fulham, netting three times in 20 league appearances before being shipped out on loan to Nottingham Forest last season.
"Nobody came in for him," he told reporters. "Sunderland played at it, played at it and played at it but it didn't come through. Unfortunately that's where we were left.
"I was a bit shocked by it but that's the brutal reality of it. Nobody came for him, unless I missed them! I wish him the best of luck no matter what people think and all the rest of it."
Former Scotland international McCormack has been tipped to join A-League side Melbourne City in the coming days to reignite his career.