Diego insists Juventus general director Giuseppe Marotta is to blame for his exit in this summer's transfer market.
The new Wolfsburg signing returned to the Bundesliga at the end of last month, just one season after joining the Turin giants from Werder Bremen.
The Brazilian playmaker signed for Steve McClaren's side for a club record transfer fee of €15.5million (£12.9m), €10million cheaper than what Juve paid for his services in 2009.
And the 25-year-old former Santos star reckons Marotta, who took up the position at the end of last season when he left Sampdoria, is at fault for letting the Serie A outfit make huge losses on some high-profile players.
"The transfer market campaign made by Juve is full of mistakes," Diego told Sky Italia. "They have lost so much money as well as stars.
"I hope that in the future they will have a winning project with the understanding that the quality not only belongs to Italian players and that you cannot exclude foreign champions. You cannot win like that."
Diego added: "(Juve coach Luigi) Del Neri was very clear with me, he said he wanted me to stay.
"On the other hand, Marotta was not clear. I always said that I wanted to stay and the coach always told me that he would have played me.
"If Marotta wanted to sell me he could have done it two months ago (instead of three days before the final day of the transfer window) as it would have been simpler and he would have found a better solution.
"The fact is that Marotta did not behave well with me or with other Juventus champions like (Mauro) Camoranesi or (David) Trezeguet. He does not have the experience to guide Juventus.
"Juventus should have been much stronger. The players that have left would have been useful to the next technical project."
Juve, who began the Serie A season last week with a 1-0 defeat to Bari, sold long-serving French striker Trezeguet to promoted Primera Liga side Hercules this summer while Italy international Camoranesi was sold to Stuttgart.
Italian pair Alberto Aquilani and Fabio Quagliarella were the only significant acquisitions made by the Bianconeri this summer.