I think it would be a mistake to start Sturridge over Firmino.
Granted Firmino has not been to his usual standard however benching him for Sturridge will change the dynamic of the team. Sturridge is not going to drop back, chase and harry like Firmino and if he does try to, he'll inevitably get injured sooner rather than later.
We simply can't trust Sturridge whilst we have better options available. Best to get games won and then look to bring Sturridge on when he doesn't have to run and press players. Whilst the team is doing well it will only be a matter of time before Firmino gets back into the swing of things.
Firmino went down for cramp, he is tired after a long season with us and Brazil. I prefer we start Sturridge who is on form then lose an out of form firmino to a hamstring or a more serious injury. We need to manage him wisely before he falls similar to De Bruyne.