Unspectacular though it may have been, the five-time Ballon d'Or winner's first goal delivered a palpable sense of relief around the Allianz Stadium, where the host had struggled to create chances against its in-form visitor.
His second smacked of a man settling into life in Turin, the former Real Madrid star coolly drilling the ball home after 65 minutes.
A memorable hat-trick seemed certain when a looping cross found Ronaldo unmarked at the far post, but the Portuguese side-footed his volley narrowly wide before missing another gilt-edged opportunity eight minutes from time, and events turned sour when Costa headbutted Federico Di Francesco in the build-up to Khouma Babacar's consolation and then spat in his face.
It looked set to be a difficult afternoon for Ronaldo when he missed from just inside the box after six minutes and fluffed a header moments later, his frustration quietly matched by an increasingly underwhelmed home crowd.
Andrea Consigli ensured the game remained goalless at the break with a reaction save when Marlon Santos turned Joao Cancelo's cross towards his own net, the goalkeeper doing well to palm the ball away.
The second half was five minutes old when Gianmarco Ferrari attempted to head the ball out for a corner, but instead hit his own post and Ronaldo was on hand to poke the ball over the line from a yard out.
With 25 minutes left to play, Emre Can's pass found Ronaldo in space on the left and he strode confidently into the box before rattling the ball beyond Consigli into the bottom-right corner of the net.
Babacar was allowed a free header to pull one back in stoppage time, but it was Costa's behaviour and subsequent dismissal that really took the shine off Juve's celebrations.