The 23-year-old was kept in hospital overnight on Tuesday after Ferguson said he was hit on the head with a ball during a training session.
However, Rafael Ortega, the doctor at previous club Chivas de Guadalajara, has suggested the Mexican striker may have a more serious problem.
"I remember in his time as a youth player at Chivas he suffered from acute migraines. Originally it manifested itself as a headache but afterwards the symptoms disappeared," he said.
'Chicarito' is likely to be out for two weeks, making him unlikely to play in the Community Shield against Manchester City on Sunday, August 7.
The Red Devils' next pre-season fixture is against Barcelona on Saturday at the FedEx Field in Washington.