Ronaldo was top scorer for the Red Devils last season with 24 goals and started the game against the LaLiga side on Sunday, but he made way at half-time for 20-year-old Amad Diallo.
The Portugal international was then pictured leaving the stadium midway through the second half, in a game which United ultimately ended up drawing 1-1.
The former Real Madrid star has reportedly told United he wishes to leave in order to play Champions League football, something the Red Devils cannot offer after they only finished sixth in the Premier League last season.
However, a number of big European clubs have distanced themselves from signing Ronaldo, with the likes of Chelsea and Bayern Munich seemingly not interested in signing the 37-year-old.
Cristiano “The King is back Sunday” Ronaldo didn’t even wait for the final whistle to leave the stadium 😭😭😭 pic.twitter.com/EWKRYXU0MI— Madrid Fan (@cristiano_peak) July 31, 2022
He had been absent from United's pre-season until last week, and his early exit from Old Trafford only added fuel to the fire ahead of their Premier League opener against Brighton and Hove Albion on Monday (AEST).
It has been reported that Ronaldo was not the only player to leave the stadium before the weekend game ended.
Ten Hag was clearly not impressed by the premature departures, saying: "It is unacceptable for everyone.
"I am telling them it's unacceptable. We are a team. A squad. Until the end we stay together."
United's points total of 58 in 2021-2022 was its lowest in a Premier League season, but new boss Ten Hag will hope it is strengthened by the addition of defender Lisandro Martinez from his former club Ajax.
Creative midfielder Christian Eriksen and Netherlands international Tyrell Malacia have also been acquired, though the Frenkie de Jong saga with Barcelona continues to rumble on, with Chelsea now reportedly interested.