Grujic is absolutely huge.
And I would love to give Allen a new contract. I could be wrong but I would guess that Klopp will use 4-2-3-1 as it's his favoured formation and he's in his first full transfer window. The thing with being in a two man midfield is that you need confidence, you need to be forward thinking, and most importantly you need the composure to be able to keep the ball under pressure. Allen is not the best at it but he's able to do it. Emre Can is understandably a little inconsistent but he too is able to do it. Henderson more often than not cracks under pressure and his first thought is to always go backwards.
This is a random example from the first Dortmund game, ironically involving Can. (idk if this will work)
[video="youtube;0yPV6NAhRCc"]https://www.youtube.com/watch?v=0yPV6NAhRCc[/video]
So whilst Henderson can do a job in a 3, if we were to predominantly play a 2 man midfield I'd be really disappointed if we sold Allen yet started Henderson. I personally want another midfielder (Dahoud) but would be much more confident with Allen coming off the bench than Henderson. And some people (not necessarily on here but on the likes of Twitter) and saying things along the lines of Allen having a few good games, or a purple patch. I'm not sure how consistently performing in the time he's had since Klopp came in October can be seen as a purple patch, just my view.