Liverpool fans like to blame the lack of squad depth but I think Klopp is symbiotic with that problem. For years he would stick to an almost identical XI every single game, until someone got injured/suspended/needed a game. Just think of the regular Liverpool side for most of the last few seasons - it rolls off the tongue: Alisson, Alexander-Arnold, Van Dijk, Matip, Robertson, Fabinho, Henderson, Wijnaldum, Mané, Salah, Firmino.
Over time Wijnaldum has been replaced by Thiago, Jota (and recently Diaz) has sometimes come into the front line and Gomez or Konate have played in place of Matip. But at least eight of the XI I just listed were starting pretty much every single game when fit. For years.
What I'm getting at is, Klopp isn't very good at rotating. He tends to prefer to play the same XI every game if he can. So why should the owners splash out on good squad players if their only role is to wait for someone to get injured and then step in? Tsimikas is perfect because he's cheap and won't mind not playing much. Origi was the same. A better backup left back or striker would demand that Klopp actually... you know... rotate his XI.
That's why injuries hit them so hard, because when guys like Van Dijk and are missing from that side they literally have no idea how to cope with it.