Last season's FA Cup finalists flew to Los Angeles on Sunday, but Sanchez was a notable absentee, with United later confirming that the player was left in England because of "personal administrative issues".
United have triggered a clause in the defender's contract that will keep him at the club through 2018-19.
The club revealed that "a personal administrative issue" had prevented the forward from travelling to the U.S., while it's since been reported by the Times that Sanchez was denied a visa due to a previous tax fraud conviction from his time in Spain.
Sanchez agreed to repay the sum in full, accepting a 16-month sentence, and he avoided prison as Spain's laws allow first-time offenders sentenced for less than two years to serve outside of jail, providing they aren't charged with another offence during the period.
Jose Mourinho surprised Sterling picked ahead of Rashford at the World Cup
The Italians would surely demand big money for the World Cup star, as would Rebic's current club, Eintracht Frankfurt.
Meanwhile, NAIJ.com had earlier reported that Mourinho criticized Raheem Sterling's performance in the third place match against Belgium at the World Cup.
The Red Devils play further friendlies against Major League Soccer side San Jose Earthquakes, AC Milan, Liverpool and Real Madrid - the last three as part of the annual International Champions Cup tournament.
United are trying their best to sort things out with the USA government, but this is taking longer than expected.
Newcastle legend Shearer: Superstar Mbappe has the lot
The forward, who regularly plays for Paris Saint Germain, at the tender age of 19 helped his nation's side go through to the quarter-finals to be played this Friday against Uruguay.