The 23-year-old, nicknamed Chicharito or "little pea", was injured during training at the Red Bull Arena in New Jersey ahead of United's friendly with the MLS All-stars, the club said on Wednesday.
"Chicharito sustained a mild concussion during training last night," a United spokesperson said on the club website.
"We have taken all the necessary precautions, which included an overnight stay in hospital.
"The manager will give further updates in due course."
Hernandez, who has 29 international caps, scored 13 Premier League goals in his debut campaign after signing from Mexican side Chivas de Guadalajara for just seven million pounds.
He is expected to be out of action for around two weeks.
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