Real meet Bayern in Germany for the first-leg of their Champions League semifinal.
De Leon was signed earlier this year, but isn't registered as he arrived after the winter market closed.
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Ancelotti explained: We want 20 of us in training and in recent games we've called on Jeremy a few times because having spoken to Ral, he's been very good for Castilla and he's a disciplined lad.
"He adds something to our training sessions because he's a young kid with real quality."