Ronaldo will claim the headlines, but Costa will be under intense scrutiny in the coming days and weeks after three appalling incidents in the closing minutes of the game. "I'm happy. I'm working hard and my teammates are really helping me adapt to this league".
Ronaldo has won the Champions League five times, including each of the last three seasons with Madrid, and one of Juve's prime reasons for signing him was so they could win the competition for the first time since 1996.
Ronaldo was reported to have been left "angry" by Real's decision to part with Ozil, telling Portuguese colleagues that the German was "the player who best knew my moves in front of goal".
Ronaldo could easily have had a hat-trick but missed two more good chances and had a penalty appeal turned down.
The Brazilian was red-carded and will probably face a lengthy ban after spitting into the face of Sassuolo's Frederico Di Francesco.
Roma was made to pay for its complacency as it fumbled a two-goal lead to draw 2-2 against Chievo Verona.
Ronaldo ended up bagging a brace to get up and running in style, while Costa was sent off for unsporting behaviour which included spitting at an opposition player.
Poland worldwide Krzysztof Piatek scored his fourth goal in three matches to give Genoa a 1-0 win over Bologna, which had midfielder Erick Pulgar sent off in stoppage time for a harsh tackle on Romulo. It's @juventusfcen 1 - @SassuoloUS 0.