So gabby's formed has dipped and we are missing half the team so lets leave out our best striker? ok then
The decision is not as easy as you seem to think. As I said, there is a reason why Gabby has started almost all games away from home when available. Again, have you thought what the reason might be? Pep being foolish?
Sergio and Gabby give us different things. I hope whoever starts has one of the best games in his career. I trust Pep. It is easier for him to pick Sergio since, if things go wrong, few would criticize him for starting a legend.