Before the match at the Santiago Bernabeu, the Liga leaders' squad wore t-shirts saying "Get Well Soon Muamba" and "Animo Abidal".
Former England U21 midfielder Muamba, 23, collapsed during the FA Cup sixth-round match at Tottenham on Saturday evening.
He was treated on the pitch and taken to the London Chest Hospital, where he remains in a critical condition.
The former Arsenal and Birmingham player's heart had stopped and he is fighting for his life.
Abidal, meanwhile, confirmed he will have a liver transplant once a suitable donor is found.
The France defender recently underwent surgery to remove a tumour from his liver, quickly returning to action, but it has been decided to replace the organ.