The former England defender quit the League Two club just one game into a five-year £40,000-a-week contract.
Director of football Sven Goran Eriksson accused Campbell, 35, of downing tools in the middle of training.
But the player hit back: "I didn't walk out halfway through training, I had a hamstring injury.
"I haven't spoken to Sven since. Why should I? It's all a mess."