The Spain international came off the bench to score a 67th-minute winner at Old Trafford, helping the Red Devils to climb up to sixth in the Premier League table.
And the 26-year-old Red Devils playmaker was thrilled to fire the 20-time English champions to a vital three points in their pursuit of a top-four finish.
“Very happy, very important win! @manchesterunited,” Mata wrote on Instagram.
Speaking to BBC Sport after the 1-0 win, Mata said he was delighted to come on and seal three points for the home side.
“I was thinking if I come on I would try to change the game and luckily I did it,” Mata told BBC Sport.
“The manager told me I was playing on the right, I tried to make the pitch wide and come inside as well. That is what I did. The most important thing is that we won and the three points will help us to carry on this good run.
“Right now I am very happy with the goal. I have three at this moment of the season. I always try to help the team. Hopefully now in the international break we can recover our injuries.”
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news