The 24-year-old has suffered a stop-start beginning to his Blancos career and will miss the Liga side's latest European tie with a thigh problem
The 24-year-old has started just one game for Carlo Ancelotti's side since his £86 million move from Tottenham in the summer having been hampered by fitness problems.
Madrid have not confirmed whether the midfielder will be available for the club's La Liga fixture against Levante on Saturday.
"After the tests carried out today [Wednesday], the player has been diagnosed with a muscle contracture in his left thigh," read a statement from Real Madrid.
"It remains to be seen how the condition develops."