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Militao became the first signing in the second coaching tenure of French coach Zinedine Zidane who was rehired on Monday as the capital club's manager.
Roma signed Zaniolo from Inter Milan in the summer of 2018, with the Italy under-21 worldwide penning a five-year deal.
Militao has had an excellent first season in Europe, becoming a key part of the Porto squad and starting 20 league and seven Champions League matches, scoring three goals in the process.
Earlier, TUKO.coke reported that Zinedine Zidane staged a sensational return to Real Madrid barely 10 months after quitting from the same position.
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A neighbor described mass casualties inside the mosque and said he saw the gunman flee. "There were bodies all over", he said. Facebook said it had removed the shooter's Facebook and Instagram accounts and deleted the video.
Los Blancos managed to beat competition for the 21-year-old's signature as they were other clubs chasing him.
The Madrid back line needs enforcement and Militao is the right man to start the enforcement as Madrid look to shuffle to their squad.
Real Madrid president Florentino Perez has been keen on the Brazilian superstar for many years and even held talks with the Paris Saint-Germain owner Tamim bin Hamad Al-Thani last year about a transfer.
Real lie third in the LaLiga table, 12 points behind leaders Barca.