According to BBC Sport's Simon Stone, the Chilean is enduring visa issues after accepting a 16-month suspended prison sentence in Spain in February over tax fraud charges from when he was a player at Barcelona:
The Manchester club have already partially addressed the issue.
Per Jamie Jackson of the Guardian, they said Sanchez, 29, would miss the start of the tour due to a "personal administrative issue" after he was not among the group that flew to Los Angeles on Sunday.
United are set to play five matches stateside before the end of the month, starting with a clash against Club America in Phoenix, Arizona, on Thursday.
As well as his 16-month suspended sentence, Sanchez was fined €590,065 (£524,575) in February over charges surrounding his image rights at Barcelona, per Adriana Garcia of ESPN.co.uk.
He has enjoyed a first summer free of a major international tournament since 2013 as Chile did not qualify for the 2018 FIFA World Cup.
The former Arsenal man will likely have been eager to begin preparations for the 2018-19 Premier League campaign, which begins for United on August 10 with a home clash against Leicester City.
But Sanchez and United will now need to resolve his visa issues before he can join up with the squad.