French midfielder Pogba was considered one of the outstanding talents to emerge from the club's Academy in recent years but his future was consistently questioned last year as he rejected offers of a new deal at Old Trafford.
However, Pogba never really managed to break through the first team despite his obvious talent and Ferguson has lifted the lid on why the 19-year-old was not involved in first team action after blasting the teenager for a lack of respect.
Now 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."
Pogba arrived at United from Le Havre in 2009 - a move that infuriated the French side at the time.
He went on to make just seven first-team appearances for the Red Devils, all last season and all from the substitutes' bench.
Three of those came in the Barclays Premier League and three in the Carling Cup, with a solitary Europa League match in a 2-1 defeat to Athletic Bilbao.
Pogba is the second highly-rated young player to leave the club this year after the prodigiously talented but controversial Ravel Morrison was allowed to join West Ham in January.
The future of a third, Ezekiel Fryers, is unclear.
He has also been offered a new contract but has yet to inform United of his decision.
"We haven't heard anything from Zeki Fryers," added Ferguson.
"I don't know what's happening there but he's not been in touch with the club. His agent hasn't been in touch with the club and I can only assume he'll be back for training."