Dont know about starting both Coutinho and Firmino after 1 pre season game and 9 days of training.
Coutinho will probably start though, but if he isnt fit enough it will be Ibe.
Both of them having been playing in the Copa America, they will hardly need much time to get back to peak fitness and will more than likely be as far along as most of the rest of the squad. Both will be spending this week training with Benteke so it's more than likely that is an advantage over e.g. Lallana and Ibe.