The Armenian playmaker looked dangerous after being introduced as a second-half substitute at Old Trafford and scored the Red Devils' third goal in sublime fashion four minutes from time.
Mkhitaryan produced an outrageous finish from Zlatan Ibrahimovic's cross on the right which was behind him, but he somehow steered the ball into the left corner of the net.
The off-season signing from Borussia Dortmund scored for the third game in succession, although he had missed United's last two outings with an ankle injury but made a stunning impact on his return.
"All his career, he's scored beautiful goals. In this moment, there's goals, defensive work, creativity," Mourinho told MUTV afterwards.
"I'm so happy he came back after his injury and he came back with happiness. And he feels at Old Trafford, people love him – and that's a very important feeling for him."
Speaking to Sky Sports about the wonder goal, he added: "It was beautiful – it's a pity that it was the third goal and not the winning goal in a great match because that would be in Manchester United history."
Mourinho's side made it four wins on the spin in the Premier League while extending their unbeaten run to nine matches, but the Portuguese coach was overly impressed with their performance against the Black Cats.
"We played, I think, not good in the first half. David [Moyes] and Sunderland deserve credit for that. I was not happy with the lack of intensity, concentration, accuracy of passing," he told Sky Sports.
"In the second half, we were strong. The second and third goals arrived late. We'd done more than enough to kill the game [before then]."