As evidenced above, Firmino's left eye got essentially 'fingered' by Vertonghen when Liverpool played Tottenham this past weekend. He had to leave the field and head straight to a hospital. He's since not trained and spent days lying in bed blindfolded.
Hence he wasn't deemed fit to start last night - and when the cameras panned on him on the bench you could see his left eye was still half shut.
Edit:
Warning: Mind the teeth!
So blunt trauma.
i presume his vision is intact on the affected eye.
or somewhat close to intact