If Coutinho plays midfield then one of Lallana or Wijnaldum has to go.
Also you guys really need to sign a top quality centre back, no more average defenders, a proper class defender that can lead your backline, also a left back because Milner can only get up his form for so long.
GK: Mignolet
LB:
CB: Matip
CB:
RB: Clyne
CM: Keita
CM: Can
CAM: Coutinho
LW: Salah
RW: Mane
ST: Firmino
Fill in the gaps and you have a team that can potentially challenge for the PL.
Also I know that a lot of people are expecting Keita to fill that defensive midfield role but I see him as more of a #8, a box to box midfielder so I think it would help the team to have a proper defensive midfielder that is happy to just play the ball winning midfielder role and im not sure if Can can play that role and if not then signing a player like Matic would be perfect next to Keita because that would also let him burst forward at times which he likes doing and also having a Keita-Matic pair would mean you can play someone like Coutinho in the midfield and tell him not to worry about tracking back.