Agreed on the pointers raised but I can't figure where is he gonna be played.
If reports were accurate that he rejected Chelsea to avoid playing at wing back over his preferred position in the middle of the park, then it'd be hard to be guaranteed a starting spot. Still see him as back up to our current midfield 3, hopefully I am wrong!
It's better to not think of first XI. I'll explain why.
Coutinho over the last three years plays about 2,600 league minutes on average for us per season. Keita played under 2,500 minutes for Leipzig. Origi, who was our 2nd choice striker, played 2,081 minutes last season in all competitions.
There are 3,420 league minutes in a season.
720 minutes of european football minimum, up to 1,530
90 - 540 minutes of FA Cup football
90 - 540 minutes of League Cup football
We played 4,230 minutes of football last season.
There is anything from 4,320 to 6000+ minutes of football to be played next season depending on how far we get in any competition. Even conservative estimates put us over 5,000 minutes easily.
Due to injuries and rotation to prevent accumulative fatigue, at the end of the season we willl end up with a core of 15-16 players who have 2,000+ minutes played for us last season. Clear backups like Klavan for example, will like be less than 2,000 (e.g. He played 1850 last season). Then the hope would be that fringe players like Solanke, Woodburn, Grujic to have played as many minutes as possible to help their progress but also reduce the burden on the core first team in terms of accumulative fatigue.
Manchester United played a lot of football last season because of their League Cup and Europa League runs.
Here is minutes played for their squad. As you can see, they have 18 outfield players who played 2,200+ minutes. Then the likes of Shaw, Young and Romero who all played 1300-1800 minutes. If you look through it's hard to pick a first XI purely from minutes played.
In a Chelsea-like season, where your first focuses mainly on the league until the later rounds of 1 cup competition, there is a very clear first XI who played almost all the league minutes. Plus Willian & Fabregas who were the 'relievers' if you will, coming off the bench or rotating into the first XI often. So more a core of 12 outfield players. With european football they won't be able to do that plus it's unlikely they will have that luck with injuries once again.
So just by nature of Lallana's injury already, being first backup for Mane & Salah plus the likely injuries to Can & Henderson during the season that they invariably get, there will be 4-5 central midfielders and 4-5 forwards who will likely share most of the minutes played. Your aim is to be in that group which he certainly will be.