Previews: Ronaldo - I will be back soon
"Thank you to all the Madrid fans for the support they have given me during my injury," Ronaldo wrote on the club's website. "I will be back soon."
Ronaldo has not been included in the squad for Saturday's match at home to Racing Santander, and midfielders Lassana Diarra and Guti and striker Ruud van Nistelrooy are out injured.
Defender Sergio Ramos is suspended and centre back Raul Albiol suffered a knee injury in Spain's 5-1 win in Austria on Wednesday and is a doubt. Real have 25 points from 10 matches, one behind leaders Barcelona, who play at Athletic Bilbao on Saturday.
Racing, fourth from bottom, are capable of surprising Real, according to coach Miguel Angel Portugal, who was appointed on Thursday to replace the sacked Juan Carlos Mandia.
"You can't set off a revolution in one training session," he told As newspaper. "But I know that we have a squad capable of performing at a higher level than they have shown up to now."
"It's important to stop H1N1 flu spreading around the squad," Barca coach Pep Guardiola said on Friday after it was confirmed defender Eric Abidal had the virus and midfielder Yaya Toure had shown symptoms.
"There's never a good time for something like this to happen," Guardiola said at a news conference. "It's true we have important matches coming up but with the others we will get by."
Barca cannot afford to slip up in Bilbao ahead of their eagerly awaited match at home to Real on November 29, defender Gerard Pique said.
"It's vital for us to be on a good run and make our rivals focus on us and try to catch us," Pique said on the club's website.
Barca will also be without striker Zlatan Ibrahimovic, who has a thigh strain, when they visit Bilbao, as well as long-term knee injury victim Gabriel Milito. Teenage midfielder Jonathan dos Santos has been included in the squad as cover.
The achievements of Pep Guardiola's Barca side will never be repeated and it will be impossible for Real to match them, Athletic coach Joaquin Caparros said.
"Real can play well but in a different way," Caparros told Spanish radio. "They are anxious because they are not only obliged to win titles but they have to do it playing marvellous football like that of Barca."
Atletico Madrid coach Quique Sanchez Flores was able to oversee training again on Friday after recovering sufficiently from H1N1 flu, the club said ahead of Saturday's trip to fifth-placed Deportivo La Coruna.
"It was horrible being absent from training for the whole week," Sanchez Flores told a news conference.
"I am calm because the players worked well and are on the right track."
Atletico have just one win from 10 matches and are in 18th place, behind Racing on goal difference.
Valencia are not considering selling any players, including Spain internationals David Villa and David Silva, despite their financial difficulties, according to president Manuel Llorente.
"The team has to concentrate on performing well and playing," Llorente said at a reception on Thursday. Valencia are fourth on 21 points and play at Osasuna on Sunday.