Henderson being our #6 isn't the problem I think. His numbers / statistics speak volumes, he does well in that position. He might have a dip in form at the moment, but generally he has done well there. It's the players in front of him, mainly Can, that are the issue at the moment.
Credit to BabuYagu, JCB & RM on RAWK.
These are some running numbers from the game against Arsenal (where we did well) but they translate to what we are seeing in other games too: Can doesn't run as much. Either he is carrying the same injury that limited him in his play last season or he just doesn't want to put in the effort. In our system and certainly in our midfield and the way we press and cut off the passing lanes, this is a major issue.
If you look at some of the goals we conceded (Watford 2nd goal, City 1st goal, Sevilla 1st and 2nd goal, Burnley 1st goal) What they all have in common, is Can being too high up the pitch, not covering for Henderson and not tracking the runners. Is it maybe possible that Henderson recognizes the problem and is committing too much to covering for Can on the right side of the pitch, where we seem to concede most our goals from? We do see him vacating his #6 position more often than last season and the season before that. Is it maybe possible that without Lallana who is the trigger for our press, Henderson is being instructed to lead the press a little more because Can isn't able to? Might be an explanation for why we see Henderson higher up the pitch / "out of position" and why our midfield doesn't seem to offer the protection it did last season.
Biggest problem is obviously missing Lallana and Alex not yet being up to speed with the tactics. Both have that willingness to run, to track players and they have a lot more pace than Can to do it. I think our overall play and our protection of the back four will improve significantly when Lallana returns.
I can see where you're coming from with Henderson being a box-to-box player. He could play that position for other teams as a box-to-box, but not in our system. He has the same limitations as Can in that position. If you look at Wijnaldum, Lallana and now Coutinho who will be playing there, what do they have that Henderson doesn't? The ability to play with their backs towards the opposition goal, beating a man on the turn and running with the ball at their feet at pace. Imagine Henderson playing there and Matip playing a pass forward through the opposition lines. What do you think Henderson will do in that situation? I think he either passes back or sideways. He can't turn quickly enough to face the opposition goal, he doesn't have the technical ability to do so.
Those stats don't tell a great deal other than 'he runs around a lot' and is no slouch when It comes to The Running Man. For example, he may not need to rack up all those numbers in running if his positional sense and awareness to discipline was better. It also may mean that being the deepest midfielder without much protection he's naturally got to cover more ground, pointing to a flawed system. Or it may not mean any of the above.
People should air on the side of caution with stats because they only paint half the picture.
I don't rate Henderson that much as a player. However, I do think he has been hard done by, caused by the system Klopp implements.
He's equivalent to Mignolet, in that a team can carry a player when too many questions aren't being asked of it. For example, when (iirc) we had Coutinho and Lallana in midfield, or Lallana and Gini in midfield we were able to boss, and dominate games by putting our opposition onto the back foot. Henderson could just sweep up behind them most of the time.
When team's counter us with a ball over the press, or dominate us through the middle we start to look more vulnerable. I said it last season with Lallana, Gini or Coutinho in midfield that the system only leaves ONE genuine central midfielder in the team. He really needs a partner, like Mascha, Didi, Alonso etc. to help him out in the engine room. It will help our defence out to obviously.
Okay, this season he has had Can and Gini most of the time in the side - both more orthodox central midfielders. But both play ahead of him and neither are rigid in their defensive abilities. All that fluffy talk of fluidity, interchanging and roaming, sees them lose that defensive shape and discipline in midfield. Again, the system isn't disciplined instead the emphasis is on individual decision making, when to roam, flow and support the attack.
FWiW, I don't see Henderson either as a #8. He is a limited player which is why I think moving him to the #6 role suited him. After seeing Lallana, Coutinho, Gini there, and having AOC there moving Henderson is a none starter, all those players are much better than Henderson. The only thing going for him pretty much is his running ability, those fellas, can pass in tight areas without treating the ball like a hot potato, they will more often than not be progressive in their passing and take chances, they have better movement and dribbling ability along with vision, and to top it off they can score goals and they do all this better than Henderson IMHO.
IMO he's either got to make it as a #6 here, but he could do with support in that role or pack his bags like Huyton says and move back to Sunderland.