The Netherlands international has failed to deliver on a consistent basis since moving to Old Trafford from PSV in the summer of 2015.
But despite his limited playing time, Mourinho was unhappy to learn of a rival boss talking about one of his players.
“I prefer not to make [such comments],” the manager explained in Friday’s press conference.
“I think my normal answer is if somebody asks me about a player from another team, my answer is the player is not my player.
“The player belongs to another team, the player belongs to another manager, no comments at all. I think that is the ethical answer, when we are always worried about ethics, respect and rules.
“I just say that if it was me making those comments about an Everton player, I would be in trouble.”
The 22-year-old forward has netted just seven times in 55 appearances for United in all competitions and is yet to find the net in the 2016-17 campaign.