Mata was substituted less than a quarter of an hour into United's 2-1 victory over Real Salt Lake after getting caught on his ankle by Sebastian Saucedo.
Darmian suffered a knock in the closing stages of the match, but Mourinho does not seem concerned by either injury.
"Mata has a big ankle but hopefully nothing really important. I would say a few days, a week, something like that, in principle. And Matteo Darmian I don't think is even an important problem," he told reporters.