He will make an 11-hour flight to the States today and go straight into the United team for Sunday's game with San Jose Earthquakes.
The Chilean's arrival on tour has been delayed due to visa problems and he missed the opener against Club America.
United boss Jose Mourinho said: “It is not about when he can, but when he has to.
“We have no players and especially in attack we have no players. Today it was Martial and when Martial left the pitch was Mata and when Mata went to the sides it was a kid that who trained with us for the first time in this pre-season.
"No Lukaku, no Rashford, not here and they probably won't start the Premier League.
“So it is about when Alexis lands and when he lands he has to play because he trained with us five days before we depart to Los Angeles and we left one fitness coach behind with him.
“It is individual training and when he arrives he has to play. If he's coming he has to play."