McTominay left for Naples for a fee rising to £30m.
Ten Hag said: “It is a little bit mixed, I am very happy for him but it is mixed because I wouldn’t prefer to lose him.
“He is Manchester United in everything, he was so important for Manchester United.
“He was here for over 22 years but unfortunately it is the rules.
“Homegrown players bring more value and that is not the right thing to do but for everyone, for all parts, it is a good deal. Scott is happy with it, and Napoli and us.”
Ten Hag insists new signing Manuel Ugarte hasn't arrived to replace the Scot.
He said: “To make the change, it is not a comparison that you can compare those players. We had three No 10s and we didn’t have any more holding midfield players apart from one so that is the idea.”