I saw a stat that Costa, Hazard and Kante have had more appearances and more minutes per average 90 than Firmino, Coutinho, Mane and Henderson.
It's the load on them during the minutes played that is relevant.
The physical load of say 20 minutes within a structure in your own third, without the ball, just adjusting to the opponent, is very different to 20 minutes 40 yards further, sprinting to press, twisting, turning and then having to make long recovery sprints if the press is bypassed.
I spent a game earlier in the season trying to watch Mane specifically and the load on him was insane due to his task in the setup, he was practically sturridge's support striker, then making lung busting runs to press them out wide and fall back into shape deep down the right when they attacked, before doing it all over again and again.
The physical load is also relevant when it comes to seemingly impact injuries, mental tiredness is going to lead to more missteps, slower reaction times, slower to get your body into position etc. This season I haven't been able to watch us exhaustively, so refrained from comment on a lot of things; what I've seen though, Klopp's got some reassessing to do on his tactics and conditioning work.