We have enough options to get through a transition period. Milner, Wijnaldum and Henderson are still with us. We could easily play two of them and one of Keita/Fabinho. But we also need to expect Keita and Fabinho to play fairly big roles for us. I think both will be involved (pitch or bench) right from the start and gradually we'll see more and more of them. Some time in October I believe both of them will start most games.
We could, and then expect as many draws as the previous season. Unless the front 3 are at the top of their game each and every time, we need more creativity from the midfield. With AOC out, and Fekir possibly (likely?) not coming, Lallana is our best midfielder in attack, far better than Milner, Henderson or Wijnaldum. I expect Keita to start right away, same with Fabinho really, but out of the current options Lallana makes the most sense beside them. He too will get a full pre season, knows Klopp's system fully, and is the best goalscorer of the midfielders we've got. I wouldn't be surprised to see him start plenty of games at the start of the season.
But sure, we could sell him, especially if Fekir does come, but I still struggle to see why. Any new player who arrives late will take a bit of time to adapt, so I'd still rather have Lallana around for at least another season. I don't want us to "get through a transition period", I want us to challange for the title. If that is our goal, having Lallana is better than not having him, even if he ends up playing just 10 games.