I don't know why Klopp keeps introducing him as a left-sided midfielder. Throughout his career he's rarely looked effective on that side of the pitch, despite having a left-foot which is almost as good as his right.
One of Lallana's greatest strengths - like Oxlade, though not as explosively of course - is his ability to overload the right-hand side, as we saw for his cutback late on, and repeatedly throughout 2016/17. I don't think his body positioning naturally lends itself to replicating that on the other side.
I don't think he looks the same player post-injury unfortunately, but given that he made more key passes than anyone except Mané despite only playing a third of the game, I don't think his impact was as limited - or worse - as others are suggesting.
Personally, though, in this system, I think he should probably be the back-up to Firmino in the number ten role. I'd have liked to have seen Salah move wider last night to give him more time on the ball, and use Lallana and Firmino's energy, superb first touches and natural synergy through the middle together.