El Editor Platense reports Sarmiento is said to be training individually at Estudiantes rather than joining their pre-season schedule ahead of linking up with City this summer.
The 18-year-old signed earlier this year before completing the season at his parent club.
Now, Sarmiento is training alone under a regime developed by City, before he gets the green light to travel to Europe.
They say he is expected to join up with City, or one of their sister clubs, next month.
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