The Dutchman, who never made a Premier League start under Mourinho, completed his move to the French side this week, signing for a fee of £16m (rising to £22m with add-ons).
Memphis’ arrival prompted an initial outburst of enthusiasm from supporters, but the 22-year-old only managed to demonstrate his natural brilliance on a handful of occasions at Old Trafford.
And Keown, writing in a column for the Daily Mail, made it clear that Mourinho would not regret letting Memphis find a new club.
“Mourinho has got it wrong before in letting the likes of Kevin De Bruyne and Romelu Lukaku go and will have thought long and hard about Depay — although I don’t think he will live to regret selling him.
“It is not healthy to keep a player you do not rate.”
It is not beyond the realms of possibility that Memphis, galvanised by the prospect of a fresh start, begins to dominate Ligue 1 and morphs into a truly dangerous talent.
Mourinho evidently considered this possibility and, in turn, included a buy-back clause in the deal, which means that United can snap up the Dutchman once more if he ever manages to fulfil his enormous potential over the next few year.
Real Madrid adopted the same strategy to great effect with Alvaro Morata, and United fans will be curious to see how the 22-year-old develops during the next stage of his career.