However much Real Madrid coach Carlo Ancelotti insisted there was no cause for concern, the sight of Cristiano Ronaldo limping off near the end of Wednesday's Champions League match has set alarm bells ringing in his native Portugal.

The national team captain, the leading marksman in both La Liga and the Champions League this term, will need to maintain his scintillating goal-scoring form if Portugal hopes to advance to the later stages of the World Cup finals in Brazil starting in June.

However, the 29-year-old former Manchester United star had been complaining of discomfort in his left knee in recent weeks but continued playing while receiving treatment from Real medical staff.

He netted a record-tying 14th goal in the latest edition of the Champions League in Wednesday's 3-0 quarterfinal, first leg win at home to Borussia Dortmund before asking to come off 10 minutes from time.

"It's a slight knee problem and I'm not worried about it," Ancelotti told a news conference.

"At the moment it is not a worry, but if it continues we can rest him, but the player is relaxed," added the Italian.

"I took him off 10 minutes before the end and there is no reason to worry."

Real has a busy schedule in the final weeks of the season and it has a potentially tricky La Liga match at sixth-place Real Sociedad on Saturday before Tuesday's return leg at Dortmund.

The world's richest club by income still has a chance at a rare treble, although La Liga losses to Barcelona and Sevilla last month saw it slip into third place behind leader Atletico Madrid and Barca.

It plays Barca, which has beaten it home and away in La Liga this term, in the King's Cup final on April 16.

One bright spot for Real from Wednesday's game was the performance of playmaker Isco, who Ancelotti included in the starting lineup after Angel Di Maria was sidelined by illness.

Real lured the 21-year-old from Malaga in the offseason but his playing time has been limited since Ancelotti settled on a first-choice team featuring Di Maria in midfield with Xabi Alonso and Luka Modric and an attacking line of Ronaldo, Karim Benzema and Gareth Bale.

Against Dortmund on Wednesday, Isco looked dangerous on the ball going forward and scored a neat goal, Real's second, with a low curling effort from the edge of the penalty area.

His efforts may have boosted his chances of inclusion in Vicente Del Bosque's 23-man World Cup squad, although competition for the world and European champions among attacking midfielders is fierce.

"He (Isco) had a great match; he defended and won balls," Ancelotti said.

"He also demonstrated his attacking ability with a brillaint goal.

"He had a fantastic game, really helping his team and demonstrating that he has that passion to do what he needs to do in order to win."

The Real fans, who at times this season have made clear their frustration with some of Isco's below-par performances, chanted his name when he was substituted in the 72nd minute.

"I'd like to thank the Santiago Bernabeu for the ovation and the support they gave us during the match," Isco told reporters.

Cristiano Ronaldo falls over Henrikh Mkhitaryan of Borussia Dortmund during their Champions League quarterfinal first-leg match at Santiago Bernabeu stadium in Madrid on Wednesday. Ronaldo scored a goal as Real won 3-0. Javier Barbancho / Reuters

(China Daily 04/06/2014 page12)