Thing is there with Matip and VvD we're so dominant in the air at the back that perhaps Klopp feels we can occasionally compromise in that respect?
Yep, very well may be the thinking. I think he's primarily competing for the advanced MF roles though, which was where I thought he was playing yesterday.
The thing I love about Lallana above all of the things he brings is his calmness and execution in and around the penalty area. I always think back to the 16/17 season when we lost to Burnley 2-0 in the second game of the season. Klopp talked about rushing the final pass, trying to force things. That if we'd played one more ball we'd have created "real chances". I've always thought that Lallana took that on board more than anyone. He always seems to respect each part of the build-up and then pick the perfect pass at the perfect time to create a real chance - the example being Keita's one from the penalty spot yesterday. Excellent vision/awareness and execution.
Add to that his pressing, strength of running, quality on either foot, ability to take and retain the ball in tight areas, to create space and a very good understanding of the game. Fingers crossed for both him and us he stays fit. If others are keeping him out of the team because of performances rather than his availability then we'll be in a very good position.