Madrid were beaten 3-2 by bitter Spanish rivals Barca in their pre-season Clasico clash in Miami on Saturday.
Gerard Pique's 50th-minute volley settled the ICC contest as Barcelona won the first Clasico in 2017-18 and left Madrid winless after three matches at the tournament.
However, Zidane was not bothered post-match, with the August 8 showdown against United in Skopje at the top of his agenda.
"This is pre-season and I'm sure that the results we've achieved weren't what we were expecting, but this doesn't alter anything," said Zidane, whose Madrid face the MLS All-Stars in Chicago on Wednesday before returning to Europe.
"The important thing is to be ready for 8 August. I saw some good things out there, but also some things I didn't like. We've got to be patient, work hard and be ready for the UEFA Super Cup.
"I'm not bothered by the defeat. It always hurts, we don't like to lose but that's not the most important thing. We need to improve and change a few things."
It was an absorbing encounter at Hard Rock Stadium as Barca stormed out of the blocks, taking a 2-0 lead inside seven minutes thanks to Lionel Messi and Ivan Rakitic.
Madrid recovered before half-time, with Mateo Kovacic and Marco Asensio ensuring the scores were level at the interval.
But Barca restored their advantage through defender Pique five minutes into the second half to clinch the ICC trophy.
"We made a very poor start," Zidane said. "They went 2-0 up within the first six or seven minutes and that was all down to a lack of concentration, we can't go thinking that it was down to anything else. We then started to gain a foothold and played our game, we had some good spells but overall you couldn't say that this was a good performance from us.
"We didn't look to hide. We came out looking to perform well and do a good job. We face Barcelona again in a few days but this was a game of contrasts: we started poorly, we then got into the game and then we gave the ball away too easily on a number of occasions, which isn't like us."