The Bianconeri needed a late Claudio Marchisio screamer to secure the 2-1 result in Tuscany.
"We should've won this game much earlier, but lacked determination today and this is why we had to suffer until the end," said Antonio Conte's stand-in.
"Sebastian Giovinco did very well and Mirko Vucinic performed the tasks we requested. We do not have a classic centre-forward, so the midfielders have to push forward in order to score. Anyone who has seen Juve play this season knows that.
"It's true that playing every three days does weigh on the squad, but at the end of the day this Juve is showing itself to be a great team.
"We are unbeaten in 46 rounds and this season won six out of seven. Obviously we can be lacking some sharpness at times, but you cannot win 4-0 every time.
"We knew Siena were strong on the counter-attack, so we had to play in their midfield and make sure not to give away the ball."