Former boss Sir Alex Ferguson was angry with the way Paul Pogba quit the club for Juventus.
Ferguson fired two separate warnings to United about Pogba - and the Frenchman's agent Mino Raiola.
Pogba's future has been thrown into further doubt after his feud with boss Jose Mourinho escalated to new levels this week.
He initially left United in 2012 to go to Juventus, before returning in a then world-record fee four years later.
But upon his departure as a 19-year-old, Fergie said: “Pogba signed for Juventus a long time ago as far as we're aware.
“It's a bit disappointing because I don't think he showed us any respect at all. To be honest, if they carry on that way, I'm quite happy that he's away, from me, anyway."
The Scot, who led United to 13 league titles, also hit out at the midfielder and his agent in his 2015 autobiography 'Leading'.
He wrote: "There are one or two football agents I simply do not like. And Mino Raiola, Paul Pogba's agent, is one of them."
