Pogba rejected new terms at United to join Juve on a free transfer.
Ferguson has announced the formal end of his time at United, telling MUTV: "Pogba signed for Juventus a long time ago as far as we're aware."
United had presented improved terms - and a promise of improved first-team opportunities - to Pogba, but he was not swayed.
The news left Ferguson, who had nevertheless been expecting the decision, fuming.
"(It) is disappointing. I don't think he showed us any respect at all so, to be honest," he said.
"I'm quite happy that if they carry on that way, they're probably better doing it away from us."