Personally I will be just as confident with Barzagli starting, if not moreso.
I love Chiellini, don't get me wrong, I even bought this season's jersey with his name on it. I will miss his tenacity, strength and bullying (and was looking forward to the Suarez rivalry). Often times this kind of attitude is needed to make the difference in a final such as this.
However, Barzagli is a better reader of the game, which will be huge with the likes of Messi, Neymar and Suarez running at our backline. He's just better positionally. Not to mention Chiellini has been very error prone this season, especially in the CL, with all the slips and such. We can't afford that in this game, a gift to Barca would all but condemn us to defeat.
Once Ogbonna doesn't start we have just as much chance as if Chiellini was fit. We can definitely still win this game if we dig deep and play a pretty much perfect game.