The 23-year-old has returned to the Santiago Bernabeu this season after a successful year at Porto as Madrid exercised a buy-back clause in the deal which took him to the Portuguese side.
Casemiro hasn't started either of Madrid's first two La Liga games, but did make his first appearance of the season as a substitute in Saturday's 5-0 thrashing of Real Betis.
Martial in talks with Man Utd
Monaco forward Anthony Martial has been given permission by France coach Didier Deschamps "to travel to England to sign a contract with Manchester United", the French Football Federation announced on Monday.
The 19-year-old striker was given an international call-up for the first time by Deschamps for the friendly games against Portugal and Serbia.
Martial, who joined Monaco in 2013 from Lyon, has slowly risen up the French international ranks having played at all youth levels.
A statement on the French Football Federation website read: "Anthony Martial requested permission from Didier Deschamps to travel to England to sign a contract with Manchester United.
"Given the exceptional circumstances, the coach has agreed to the request of the player."
Martial is due to return to the French training base at Clairefontaine, south of Paris, on Tuesday morning, the FFF added.
Reports have said that Manchester United could be willing to pay up to 50 million euros (£36 million, $56 million) for Martial who cost Monaco just 5 million euros when they bought him from Lyon.
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