You are right, but fortunately the only thing we really need is one more quality forward. Yes a CB would be nice, but we are ok'ish there with the cover of Fabinho, plus maybe Lovren remembers how to defend again some day.
I would also add that unless Klopp and the rest of the staff are 100% into Neco Williams and consider he versatile enough to deploy as a RB and LB (this would be ideal) then I think Left-Back would be a vital position we need to buy cover for. The two full-backs we have literally shape the way we play. When Robertson is not playing we only have Milner and this is already not a viable option.
It's not going to be an easy thing tho. I think its always tricky to buy back-up full-backs because its a position that does not get rotated often and unlike the midfield, CB and the forward roles there is only one guy playing that role, its also not a position susceptible of being subbed off too often (winning or losing, most of your changes will be in midfield and upward) and rarely do you stumble upon players as comfortable at LB than they are at RB and vice versa which ultimately means less playing time for that player.
If we go for the ''buy'' option, I imagine it would have to be an experienced LB who wont mind playing second fiddle or somebody young that will kind of be an understudy to Robertson