The 57-year-old coach was full of praise for the in-form defender and stated that Los Blancos may realise that they were wrong to sell the player
The 25-year-old centre-back signed for Benfica in the summer in a €5.5 million deal after becoming frustrated at his lack of playing opportunities at the Santiago Bernabeu.
He has become an integral piece of Jorge Jesus' squad, helping the club overcome the loss of David Luiz to Chelsea in January 2011.
"I do not know [if Madrid regret losing him], but he is doing very well in Portugal," Sabella told Marca.
"They call him Benfica's 'defensive wall', together with Luisao. But it is not the same thing as playing in Spain, the playing styles are different.
"Time will tell if Madrid made a mistake or not."