Jose Mourinho's side squandered the lead and crashed to a 3-1 reverse to Barca at their Bernabeu stadium, a result that lifted the champions back to the top of the standings on goal difference, albeit with one more game played.
Portuguese forward Ronaldo missed two excellent chances and fluffed a slew of free-kicks. He suffered the indignity of being whistled by his own fans and was slammed in the normally cheer-leading Madrid-based sports media.
The former World Player of the Year's performance again prompted unflattering comparisons with the current holder of the award, Barca's Argentine forward Lionel Messi, who had a relatively quiet night by his standards but whose brilliant run and assist created Barca's equaliser.
Roberto Palomar, a columnist in Marca sports daily, accused Ronaldo of selfishness on Monday, saying the 26-year-old was "obsessed" with winning a second World Player award and too often shot at goal instead of passing to a team mate.
"The club are going to have to bribe [Ronaldo] so that he behaves like a footballer and not like a child," Palomar wrote.
"Cristiano wants the Ballon d'Or at any cost and is so obsessed that he thinks the only way of getting it is to play his way," he added.
"And his way of playing is to shoot at goal whenever he gets the ball, from any position... whether there are team mates free or not."
NOT FAIR
Club captains Iker Casillas and Sergio Ramos have rallied behind Ronaldo, who became the world's most expensive player when he joined Real from Manchester United for a fee of around 90 million euros at the end of the 2008/09 season.
"Here [at Real] we all win and lose together," Ramos told reporters after Saturday's match.
"It's not fair to single out the guilty or blame the defeat on one player," added the Spain defender.
"Cristiano is a great player and he always helps us."
Three of the Cup last-32 first legs have already been played, all featuring top-flight teams at third-tier opponents.
Barca, who lost 1-0 to Real in last year's final, won 1-0 at Hospitalet, Atletico Madrid were beaten 2-1 at Albacete and Athletic Bilbao won 1-0 at Real Oviedo.
Four of Tuesday's first legs are all first-division clashes, with Getafe hosting Malaga, Granada playing at Real Sociedad, Racing Santander at home to Rayo Vallecano and Sporting Gijon at Real Mallorca.
The return legs are next week.
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