Ten Hag, on Mount's goal in the draw at Brentford, told MUTV: "I was so happy for him.
"But so sorry for him also that it wasn't the winner.
"He deserved that moment of glory, maybe it was such a small moment but, for a player who came in and suffered so many injuries and worked so hard all the time, and had so many setbacks, it was so frustrating.
"And then you think you've scored a winner so we were disappointed when we conceded that goal that makes it 1-1. It makes a totally different mood, as you can imagine."
Ten Hag also stated: "I don't think they (Chelsea) wanted to sell him. They wanted to keep him and they even offered him a new contract many times.
"But he wanted to make the step and we were, and we are, very pleased he is a Man United player because he has great abilities.
"I am sure he will contribute and he will become a big player for Man United."