Does he have a) the lateral mobility required in our set up
The ability to move left and right? I'm pretty sure all players have that
b) the defensive ability on the ground
Aren't his pressing numbers quite high? And didn't he have good tackling numbers for an attacker, since his Southampton days?
c) the aerial ability defensively .... because in our set up the 6 as currently played needs all these 3.
The height thing is a Klopp preference. Our set up doesn't demand it. If you prevent the long ball forward anyway, then there's no real need for aerial ability. But even at that, you're really referring to "height", rather than "aerial ability" though. Lallana probably falls down on that, but it's not a requirement for a #6 - it's just a handy thing to have (height, I'm talking about). Makelele was the perfect #6, and he was a short arse
He can retain possession well we know that and you could play him there in specific game conditions - vs a low block team with no counter attack threat ... beyond that idk what he brings to the position?
(And we’re assuming he’s match fit in this analysis which he hasn’t been for 2 years so it’s all a bit hypothetical)
He would bring the ability to carry the ball into the attack, the ability to switch play under pressure, the ability to receive from the defenders, and the ability to replace the fullbacks when the other team counters. Plus, a two-footed #6 opens up the field in a way that a one-footed one doesn't.
He certainly wouldn't be the BEST option at #6, but to say he doesn't really have the tools for it would be an underestimation, I think. As you say, it could be that he'd be more suited to games where we don't have to defend too much. But then again, those are the games where we probably do need some sort of creativity in that position, in order to maximise the attack and get us the goals we need to break the deadlocks that probably scuppered our league challenge last season.