Amauri was the matchwinner for Juve, converting a free-kick from fellow Brazilian Diego.
"Today it was crucial that we get a victory - a deserved one - with few real scoring opportunities, but never risking and always keeping control," said Ferrara.
"I am satisfied with the result and the performance. We can then debate on the quality of football at a later date. The tempo was pretty slow and it allowed Siena to clam up. We kept the ball, but passed it around very slowly and without the right timing, especially in the first half.
"After the break with the substitutions we found the right push to go for the victory. Is the glass half-full or half-empty? It's completely full."
On Diego, Ferrara added: "He is certainly no longer affected by the injury.
"Today Diego, [David] Trezeguet and Amauri had a few problems figuring out the space and movement needed."