MADRID, Dec 17 (Reuters) - Real Madrid's four-point cushion over Barcelona at the top of the Primera Liga leaves them free to take risks when they travel to face their arch-rivals next weekend, said striker Ruud van Nistelrooy. Second-placed Barca had closed the gap to just one point with their 3-0 win in Valencia on Saturday, when returning Cameroon striker Samuel Eto'o struck two first-half goals. But the champions responded with an unimpressive, though ruthlessly efficient, 2-0 win over Osasuna at home on Sunday. 'It was important to win so that we can go to the Nou Camp with a four-point advantage,' said Van Nistelrooy, who scored the opening goal. 'With this we can go there to attack, without fear. It will be a tough game and it is better to go secure in the knowledge that whatever happens we will remain leaders.' Barca put on one of their best away performances of the campaign to beat free-falling Valencia, but were dealt a major blow when Lionel Messi picked up a thigh injury that rules him out for around a month. The Argentine international is their top scorer in the league with eight goals, and notched a hat-trick against Real in last season's 3-3 draw at the Nou Camp. 'The loss of Messi is very important for Barca because he's a key player in fantastic form, but they have recovered Eto'o who has proven beyond doubt he is well again,' said Real goalkeeper Iker Casillas. Barca coach Frank Rijkaard agreed with Casillas's assessment in an interview with Com Radio when he praised Eto'o, who was sidelined for the first three months of the season with injury, as something akin to Barca's missing link. 'The victory against Valencia was very important psychologically,' the Dutchman said. 'Eto'o's quick touch and penetration created problems for Valencia. It's what we want great goals and attitude. Now we have the chance for a good season.'