It's not about doubting him. It's about having reasonable expectations. He's a great player, but to expect him to score nearly 20 goals in his debut season is fantasy. I'd love it if he proved me wrong and did it. But if you're asking me to me to make predictions, I'd rather be cautious than be a dreamer.
My expectations for him plummeted after the Tevez deal. Why? Because im pretty sure that Tevez will complement everyone in the team, he is just too versatile and hardworker. Defenders will have a headache marking tevez and llorente will get a lot of space to run and finish plays. Now, if we take in consideration that Llorente doesnt needed much space in the first place because he is accustomed to be played as a lone forward, and having Tevez get a lot of manmark from him will make Llorente´s probs of failing much much less.
Both are physical players and Llorente for being a prima punta, is considerably technical. Tevez is both technical and physical. It will be hell to mark those 2.
All this considered, the probability of those 2 working well together are high IMO.