Real Madrid goalkeeper Iker Casillas has quashed speculation that former Manchester United playmaker Cristiano Ronaldo has become unsettled in Spain.

Last month, the Real Madrid star was subjected to jeers from the notoriously fickle Bernabeu crowd after the forward missed a number of chances against Barcelona.

And the boos continued at the weekend as fans registered their displeasure with the Portuguese international, despite the ex-Manchester United player having scored 21 goals in 17 league games this season.

"Cristiano Ronaldo is a very competitive player who always tried hard and wants to do his best. People will always ask a lot of him because of the sort of player he is," Casillas is quoted in the Daily Mail.

"He wants to do his best for himself and for the club. The Bernabeu crowd is very demanding, and that's a good thing.

"There is no debate about his quality. He has always had the support of the fans."

The 2011 Fifa Ballon d'Or runner-up, who equalled the Spanish league record of 38 goals in a single campaign last season, has struggled to endear himself to sections of the club's support since his world-record transfer from Manchester United in 2009.

Since his departure, Ronaldo has been consistently linked with a return to Old Trafford. Recent income tax laws in Spain, whereby foreign players must now pay 54 percent since the abolishment of the 'Beckham Law', has heightened rumours suggesting he could endeavour to return to Manchester United in the near future.

When David Beckham left United in 2003, elite foreign workers were only required to pay 24 percent tax, but the abolishment of that rule, which was named after the Old Trafford icon, has certainly made it a more equal playing field.

The Sun reported last week that the precarious new financial rules, which would have a serious impact on Ronaldo's reputed £250,000 a week wages, in addition to the winger's well-documented fragile relationship with supporters, has led to both Manchester United and their bitter Premier League rivals Manchester City being linked as possible suitors.