Lisbon: Portuguese doctor Nuno Campos, a former member of the medical team of his national football squad, has estimated superstar Cristiano Ronaldo could be out of action for 18 days if he has a muscle strain.

Real Madrid's star striker Ronaldo was injured in a La Liga match and had to sit out his team's frustrating 0-0 draw against Manchester City in the first leg away match of the Champions League.

Without Ronaldo, Madrid' attacking potency reduced considerably with the Spanish giants unable to breakthrough a disciplined Manchester City. Worse, they at times looked blunt and were visibly lacking the creative spark and penetration that Ronaldo ensures.

Ronaldo is the leading scorer in Champions League this season with 16 strikes, way ahead of FC Barcelona's Luis Suarez (8).

Media reports in Spain suggest he is suffering from a muscle problem.

"If the diagnosis really was a muscle strain, recovery lasts between 18 and 21 days. Twelve or 14 to heal completely and another five or seven days for muscle recovery," Campos told the Portuguese sports newspaper Record on Thursday.

Campos explained that in the case the injury was just a muscle contracture, it would not prevent Ronaldo from facing Manchester City on Wednesday in the second leg of the Champions League semi-finals.

The expert stressed that the most important response with muscle strain would be to not force Ronaldo to rejoin his team as soon as possible, as the player would then risk being unable to play in the Euro 2016 championship.