Napoli were trailing to the Serie A champions 2-1 (5-2 on aggregate to Juve) when he came on for Arkadiusz Milik in the 60th minute, but eight seconds later the Belgian had levelled things on the night.
Mertens sprinted onto the pitch and read a mistake by Juventus goalkeeper Neto, who miscontrolled a simple pass and allowed his adversary to tap the ball into an empty net.
Napoli would go on to win the match 3-2, but exited the competition after losing 5-4 on aggregate. Mertens' goal was his 26th in 38 appearances for Napoli this season.
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