Angel Di Maria played for Argentina in the Copa America this summer
Argentina international Di Maria was due to fly from Buenos Aires to San Francisco to join United's US tour squad.
But neither player arrived. Rojo's absence was explained as a problem with his passport but Di Maria's no show was left unexplained.
United boss Louis Van Gaal said after the 3-1 win over Barcelona: "Rojo was not on the plane. He has passport issues, but for Di Maria, I don’t know why. "
Reports in France say Di Maria will have a medical with PSG and complete his £45million move by the time the two teams meet in Chicago on Wednesday.
United officials continue to refuse to confirm or deny a deal has been struck with PSG.
Mexico international Javier Hernandez did arrive here after recovering from a collar bone injury.
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