Firmino being a doubt is most likely because he looks exhausted. If he's unfit, leave him on the bench. If he's fit, only an idiot wouldn't play him. But, that's assuming he's playing upfront. He's wasted and pointless on the wing. Which I wouldn't mind so much if he were making way for Suarez. But, Origi isn't quite at Suarez's level...
Start with studge. Lallana is a no brainer. Firmino can come on for studge on 60, or 85 if Klopp does his usual.
Lucas should have been rested at home and played for the weekends away game. Lord knows why he was started against the mighty saints.