United warmed up for the showdown with the Champions League winners by beating Sampdoria 2-1 in a friendly in Dublin, with Mata scoring the winner from an Anthony Martial cross.
Victory at Aviva Stadium was United's sixth in their seven pre-season fixtures, with Mata saying that the whole squad was feeling positive heading into the new season.
"It was a great pre-season," the Spaniard told MUTV. "We had a few amazing weeks in the States playing some very good rivals and against Sampdoria it was difficult to beat this tough Italian team."
Mata said he is now firmly focused on adding the Uefa Super Cup medal to his collection after missing out on winning the trophy twice during his Chelsea days.
United beat Super Cup opponents Real Madrid on penalties in a friendly in Santa Clara last month, but Mata said he was braced for a tough match against the reigning Spanish and European champions.
"I've been lucky to have played in two [Super Cups] and unlucky to have lost two, so maybe it'll be third time lucky," he said.
"Last season was a good season, now we have an opportunity this season to lift more [trophies]. Next week we have the chance to lift another one which is going to be difficult against a team like Real Madrid. Our minds are now focused on that."
The former Chelsea and Valencia playmaker also talked up the pace of United's forward line and the understanding between him and Martial after the two combined for the winner against Sampdoria.
"That's ideal for players like me as we try to look for passes for the quickest players," Mata stated.
"We all know how good Anthony [Martial] is when one-on-one with a defender, so I just tried to put myself in a good situation to finish. It was an important goal because we won."