Former Brazilian international Ronaldo, who played for both Real Madrid and Barcelona, believes Real will win the Champions League semi-final first leg between the two sides on Wednesday.
"My feeling is that Real Madrid will win the match," he said during an interview Tuesday with Brazil's Sport TV which was picked up by the Spanish media.
"I commented last week with some friends that Barcelona are the best team of the last 15 years. All the players in all positions know exactly what to do both when they have the ball as when they don't," the 34-year-old added.
"It is going to be a very difficult match. The outcome will be defined in the second game, but you never know what will happen in a 'clasico'."
Ronaldo joined Real from Inter Milan in 2002. He won two league titles, one Intercontinental Cup and one Spanish Super Cup with the club before being sold to AC Milan in January 2007. He also played for Barcelona in 1996-97.
The player announced his definitive retirement from competitive football during an emotional press conference on February 14, citing fatigue and a hypothyroid problem which played havoc with his weight.
He is one of only two players to win the FIFA Player of the Year award three times, the other being France's Zinedine Zidane, who also played for Real.
Ronaldo will bid his final farewell to football in a friendly against Romania in Sao Paulo on June 7.
Real face Barcelona at their Bernabeu stadium on Wednesday for the first leg of the Champions League semi-final. The two sides will face each other in the second leg on May 3 at Barcelona's Camp Nou stadium.