The teenage midfielder will leave River Plate for City at the end of the year.
Ortega said: "He has an impressive talent.
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"He is one of those players who makes me like him because he aims and dribbles. There are few like him.
"I love him and I hope he can make the River fans dream."
He added: "What Marcelo (Gallardo) did (for River) was incredible: he won everything in just a few years. He brought River back to where it deserves to be and we have to thank him for all the joy he gave us as fans."