The 37-year-old Portuguese forward made an official request to leave the club in the summer after they missed out on Champions League football last season. But he missed out on a move before last month's transfer window closed.
So far, Ronaldo has been a bit-part player for new United boss Erik ten Hag, with the Dutchman preferring to start with Marcus Rashford up front in the past four matches – all of which the club have won.
Ronaldo's only start so far this season was in the 4-0 humiliation away to Brentford in August.
Ten Hag is likely to rotate his squad for Thursday's game against the Spaniards, with new £60 million signing Casemiro set to partner fellow Brazilian Fred at the base of midfield.
Club captain Harry Maguire, Victor Lindelof and Aaron Wan-Bissaka are likely to start the game but Luke Shaw and Donny van de Beek were absent from Wednesday's training session at Carrington.
New signings Antony, Tyrell Malacia, Christian Eriksen and Lisandro Martinez also took part in training.
You can see the Manchester United squad put through their paces in the gallery above. To move to the next image, just click or swipe left.