Manchester United kicked off their pre-season tour with a 1-1 draw against Club America on Thursday night.
Previously, at a press conference in the United States, the boss also discussed the Chilean in his plan for the start of the Premier League campaign.
"So I'm anxious because I'm not training [with everyone] and then I go to the Premier League without lots of players".
"Valencia will be the captain", Mourinho is quoted as saying by ManUtd.com. "For Paul, I think to be world champion can only be fantastic". "I think he's going to be the captain and when he doesn't play, we have to make decisions".
"It depends on who is on the pitch".
The Independent is reporting that the Red Devils are still keen to try and conclude a deal to bring the 29-year-old to Old Trafford this summer as Manchester United look to bolster their options at the back ahead of next season.
"But I never cared much about the captaincy, I care more about the life in the dressing-room and the leaders".More news: Homeland's Nielsen: No Doubt Russia Tried to Interfere
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Mata got United's first goal of the summer with the equaliser against Club America.
Chong joined the Premier League side from Feyenoord in 2016 and has been part of the Under-18 side since.
Manchester United have confirmed new signing Diogo Dalot will miss the first month of the new season with injury.
"Fred, you know we did it quite a long time ago, we started the process a long time ago", Mourinho said in Los Angeles.
"He has to play where he belongs, he has to play in the Academy and he has to work physically because physically is where he's not ready".
Both sides had to contend with hot conditions as temperatures reached 40C, and Mourinho told MUTV: "They were a team with intensity and quality so it was a good training session for us".
"But the talent is there, the personality is there, the passion to play, the humility to learn, so this is a fantastic experience for him".