Having sold Albert Sambi Lokonga to Arsenal at the beginning of the week, Kompany is now losing his potential successor - Joseph Nonge.
Anderlecht raised €20m - plus bonuses - from the sale of Lokonga and had earmarked Ngonge, 16, as his replacement.
However, Juventus have swooped to sign Ngonge for a knockdown €200,000.
Ngonge was training with Kompany's senior squad at the beginning of preseason, but is now in Turin, having agreed a three-year contract with Juve.
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