Pretty clear what Lallana's functions are
1) He leads the high press (he's the best at it in the squad - Firmino is good too but in a different way - Lallana is brilliant at relentlessly closing the space in front of him)
2) He retains the ball - especially in the final third which are other AMs suck at
3) He transitions us from midfield to attack (again he's the best at this in the squad)
Klopp has picked him for every significant game when he's been fit because he does stuff which no one else in the squad can replicate
That's a really unique set of qualities you've described though. I actually (admittedly probably due to my own ignorance) don't know another player in world football (let alone an attacking midfielder) whose main qualities are pressing, ball retention and transitions. If you describe his qualities in that way, I would agree with you, but are those a unique set of skills that other players can't replicate, while also scoring and/or assisting more?
I'm partly playing the devil's advocate here but I really do want to draw this out a bit more. Pressing is something many of our players do. It's something that many players in many teams do. Similar for retaining the ball. Similar for transitions. Is it the combination of the three that makes him unique?
I feel like the way he's being described here almost make him sound like a latter day Lucas (back when he was still an up and coming lad), where much of the match viewing public can't quite pinpoint his value. And that may well be the case, I freely admit. He would be a difficult player to pigeon-hole. Look at his stats:
Per game he has:
1.4 shots
1.9 key passes
1.1 dribbles (which I find amazing, because he's obviously a very good dribbler)
1.9 tackles
1.1 interceptions
1.2 clearances
Looking at this stats, his attacking stats look decidedly average, but defensively he's a beast for an attacking midfielder. And yet, he looks so good on the ball that you would think that he
would score and assist and make more key passes? Is his output in goals and assists even something we want to see him improve on? Or are we happy for him to do his thing, and in doing so allow others to score and assist? Would we trade him for an attacking midfielder that scored/assisted a total of another 5-6 times a season, but who only pressed as much as say, Firmino?
All genuine questions.