Manchester United manager Jose Mourinho said Thursday's 1-1 pre-season draw with Club America in Arizona "pushed us to levels we are not prepared for".
United had to come from behind against the Mexican side as Juan Mata's 78th-minute strike cancelled out Henry Martin's opener on 59 minutes.
The Premier League side were without many key players after the World Cup.
Paul Pogba, Romelu Lukaku and English duo Jesse Lingard and Marcus Rashford were among those missing.
David de Gea, Nemanja Matic and new Brazilian signing Fred are expected to join the team next week.
Alexis Sanchez is also due to join the team shortly in the United States after resolving his visa issues.
The Chile forward, who accepted a 16-month suspended jail sentence in February for tax fraud, missed the start of the tour as he was unable to get a US visa.
Both sides had to contend with hot conditions as temperatures reached 40C, and Mourinho told MUTV: "They were a team with intensity and quality so it was a good training session for us."