Are Real Madrid fans ever happy? Just last week, they were mad at Gareth Bale for missing a couple of shots in a match Real won with relative ease. Now they're upset at Cristiano Ronaldo for having the audacity to throw a party for his 30th birthday (albeit on the night after a humiliating 4-0 loss to bitter city rivals Atletico Madrid).

Perhaps the disgruntlement stems from the fact that, after serving a two-match band for a red card, Ronaldo returned against Atleti and did little to distinguish himself. In fact, he largely went missing for most of the match. The problem for Ronaldo now is that it may not be just the fans that are getting fed up with him.

According to a report in the Catalan publication Sport, Ronaldo may no longer be considered as indispensable to Real Madrid as he once was. Also, multiple reports in Spain indicate that club president Florentino Perez may even be considering cashing in on the Portuguese star while he's still at the top of his game.

Coming from the rumour-mad Spanish sporting press, this may sound like crazy talk. But when you consider the ruthless manner in which Perez has wielded the axe in the past, maybe it's not so insane. Mesut Ozil, Angel Di Maria and Xabi Alonso were all considered indispensable members of the squad, right up until the moment they were sold.

Since joining the La Liga giants from Manchester United in 2009, Ronaldo has come to embody everything that we think of when we consider the current Real Madrid team. It's difficult to imagine Los Blancos without him. However, if the club wants to recoup anything close to the then-record $123 million transfer fee it paid for him, Ronaldo's sell-by date may be fast approaching.

Man United fans have long desired to see Ronaldo back in the colours of the Red Devils. With Real Madrid making no secret of its desire to bring in United keeper David De Gea as a successor to Iker Casillas, the idea of Ronaldo returning to Old Trafford in some sort of partial trade deal isn't entirely unfeasible.

Ronaldo, to his credit, has publicly been very quiet this week. Like all great players, he knows that ultimately there's only one place where what he does really matters: on the pitch.