Debutant goalkeeper Lee Grant made a couple of important first-half saves but there was nothing that his replacement Joel Pereria could do with Hector Martin's thumping 59th-minute header into the top corner.
Mata ensured that United's pre-season preparations would not begin with a defeat when he squeezed a 15-yard shot through goalkeeper Oscar Jimenez, after Ander Herrera's header had come back off the post.
Relive how the action unfolded at the University of Phoenix Stadium with Sports Mole's minute-by-minute updates below.
Subs: J.Pereira, Darmian, Tuanzebe, Fosu-Mensah, Williams, Hamilton, Garner, Gomes, Chong, Bohui, Greenwood
Mourinho said: Hopefully, he [Sanchez] can at least join us later because it is important to work with Juan Mata, Alexis and Anthony Martial, who we think are the players we have for the start of the season. There is no Lukaku, Rashford or [Jesse] Lingard."
He said: "I want to try to prove to the manager that I can be in his team, and not just for these games...when everyone comes back and everyone is fit and ready, I still want to be one of the first names on the team-sheet. It's time for me to show him over the next few weeks in particular what I can do and why I deserve to be in his team."
Mourinho confirmed as much to United's official website, saying: "Joel is a Manchester United goalkeeper but, two years without football, he has to play. Joel is here with us. It's good for him to play but then he will go on loan"