Kaka said: "I rejoice in the signing of Cristiano.
"He is one of the best players in the world and will help Real Madrid fight for every title next season."
Gabriel Heinze has taunted United boss Alex Ferguson over Ron's move.
The former Old Trafford defender said: "I'm sure Cristiano will be better at Madrid than he was at United.
"It's only normal he would want to come and play with a club like ours. The team will be more powerful with him.
"He's a phenomenon but he can improve his standard even more here. I expect him to do exactly that."