Lallana in midfield is just so crucial to this team, a similar backup has to be the top summer priority.
Games Lallana has played on the wing: Bournemouth (4-3 loss), Man City (1-0 win - not great performance), Man Unt (1-1 - ok performance but not fluid), Swansea (3-2 loss).
Games Lallana has been injured so Can has played in midfield instead: Man Unt (0-0 - struggled), Southampton (0-0 - again, couldn't score), Sunderland (2-0 win - we won but this was a frustrating performance).
Not once without Lallana in midfield have we been at our fluid best, so Jurgen insisting on not playing him there today is beyond me. I think similar things can be said for Firmino out wide (never at his best there), Can in midfield (slows everything down) and playing without Mane (no coincidence that we haven't won without him).
Our first eleven works brilliantly, but take one crucial piece out (Matip, Mane, Lallana, Coutinho) and there is no like for like replacement. The team is shifted too much and we lose cohesion, none more so than when we move Lallana out of midfield.