With his charges reeling 2-1 following a Duvan Zapata brace, Juve coach Massimo Allegri sent on his 100 million signing in the 65th minute.
Within 13 minutes of the former Real Madrid star's introduction, he pounced on a loose ball in a goalmouth melee to head home his 12th league goal of the season.
It was an impressive response from the Portugal international and Juve who were reduced to 10 men after Rodrigo Betancur was dismissed for his second yellow card eight minutes into the second-half.
The stalemate - only the second time Juve have dropped points this campaign - extends their lead at the top to nine points.
However second placed Napoli can cut the deficit to six points on Wednesday night provided they beat third placed Inter Milan at the San Siro.