Scott McTominay and Andreas Pereira both started the match as they look to continue their development and stake a claim for more game time in 2018/19, but with United lacking several first teamers after the World Cup, Demetri Mitchell was also handed a start in a front three.
Emerging goalkeeper Joel Pereira, who will go out on loan this season, replaced impressive veteran debutant Lee Grant at half-time, while there were also opportunities for Timothy Fosu-Mensah, Axel Tuanzebe, Angel Gomes, Mason Greenwood and the aforementioned Chong.
Greenwood, who was bright when he came on, is only 16 years of age, while Gomes, who famously made his first team debut in May 2017 is still yet to turn 18.
18-year-old Chong, whose cross brought United's 78th minute equaliser for Mata after Club America had taken the lead through Henry Martin, drew the greatest praise from the boss.
"He was good. He came with the intention to show, and in a moment where we have no strikers on the pitch, Martial was the only one and then he left, so we needed people to create, have some initiative, which he did," Mourinho told MUTV.
"The action for the cross for the goal is a beautiful action. He was enthusiastic, he was confident to play," the Portuguese added.
"Of course physically he has his limitations, and when he has one more touch and he allows that body contact of course he loses it, but when he makes the ball move and attacks people one against one, he has quality. He's a good kid."
Mata was equally full of praise, telling MUTV, "It's very good news for the club and for the academy that we have so many kids with us [in the squad] and I think they're having a great experience by training with us, with the manager, seeing what Manchester United represents.
"I think the future is bright for Manchester United with these academy players."
United's next game will take place on July 22 against MLS side San Jose Earthquakes.