If Louisito and Coutinho are not in their best form, I would go for a something new: playing Dembélé as the forward striker, running in any available spaces. He can combine with Messi and get some assists from him.
So I would go for this balanced 4/4/2:
Stegen.
Semedo. Piqué. Lenglet. Alba.
Vidal. Busquet. Arthur. Roberto (or Rakitic).
Messi (free player) + Dembélé (dynamic striker)
I think Vidal can help us in terms of physicality, workrate, running the field, helping Semedo to close the flank against Mané and Robertson.
Arthur is also important to help Busquets in ball circulation and ball retention against high pressing.
Dembélé for the speed and the attacking threat + Messi our creative free player.
I'd go for Roberto + Vidal over Rakitic, because these two players are more versatile, more dynamic and can help in attacking transition.
I think our fullbacks will be more "cautious" and defensive than usual because Mané and Salah and very direct and fast in counterattacks.
Suarez and Coutinho are slow, not in great form, lose the ball easily, so I won't see them suitable for this particular match, but they could help in the second half.
Anyway, I'm not the manager to pick the gameplay. I will trust the team and let's hope we take the win. Visca Barça !