Real Madrid have been hit by a huge injury scare ahead of their crunch Champions League quarter-final against Bundesliga champions Bayern Munich. Goalkeeper Thibaut Courtois has been sidelined with an injury and is expected to spend several weeks out of action. Back-up goalkeeper Andriy Lunin will take over from the Belgium international while Courtois is out.Courtois picked up the injury during Tuesday's Champions League last-16 second leg against Manchester City. The goalkeeper was replaced by Lunin at half-time of the game as Real completed a 5-1 aggregate win to book their place in the quarter-finals of the competition. Real Madrid manager Alvaro Arbeloa said after the win that the substitution was only a precautionary measure. He told reporters: “Courtois had some discomfort and wanted to continue, but it wasn’t necessary to take any risks because in four days we have another final: a very important derby and a very challenging match. The good thing is also having a goalkeeper like Lunin, who has been frankly good in the second half and has been solid. From here, I want to congratulate him because what he does every time he gets the opportunity to defend Real Madrid’s goal isn’t easy."However, Real Madrid have now confirmed that Courtois does have an injury and is out of action: "Following tests carried out today on our player Thibaut Courtois by the Real Madrid Medical Services, he has been diagnosed with a muscle injury in the rectus femoris of his right quadriceps. His recovery will be monitored."Real Madrid are expecting Courtois to miss the next six weeks, according to Diario AS. If the report proves accurate it means that Courtois will sit out some crucial fixtures for Los Blancos, starting with Saturday's derby against Atletico Madrid in La Liga. Courtois is also likely to miss the two Champions League games against Bayern as well as La Liga games with Real Mallorca, Girona, Alaves and Real Betis.Lunin will step up while Courtois is sidelined and is an experienced option. However, losing Courtois will be a massive blow for Arbeloa's side, with the goalkeeper widely considered to be one of the best in the world.Add GOAL.com as a preferred source on Google to see more of our reportingMadrid may have to cope without Courtois, but Arbeloa is still feeling of confident when it comes to the Champions League. He told reporters after beating City: "The Champions League is special for Real Madrid, and we’ve faced one of the best in Europe. And then Bayern and perhaps PSG later. If we want to win them, we have to play like tonight and as we did six days ago. We’ve had some luck in these two matches, but we deserved to win both. The aggregate score of 5-1 says it all. We have to keep moving forward, and surely we’ll face Bayern.”Real Madrid host Atletico on Sunday night at the Santiago Bernabeu. Diego Simeone's side won 5-2 against Real earlier in the season in La Liga and head into the match after booking their place in the Champions League quarter-finals by beating Tottenham.