Laporte's injury hurt us a lot - but we were weak at LB too and we lacked (and still do lack) a destroyer at mid (Fernandinho had to play CB quite a bit last year so we didn't have a midfield destroyer ahead of us).
This year: our 433 backline of Someone - Laport + Dias + Walker is far better than we had last year. Still need a LB and a ball-winning destroyer at mid though.
I mean, my point is that even though we may have issues at LB and DM (I disagree on that but that's another discussion), the only actual relevant part is the CBs.
We had no LB for much of 17-19, and in both seasons there were spells we didn't have Fernandinho. But we were fine. In fact we won a shit ton, even with Zinchenko at LB.
Last season, everything went to shit without Laporte. It exaggerated all of our other issues: LB, Silva + Aguero ageing, Fernandinho in defence, weakening press etc. But firstly, we were actually looking really strong in the games prior to Laporte's injury, and secondly we looked vastly improved after he came back. We still had some issues as Laporte couldn't do it all but a lot of problems were lessened just by having him back.
Now we have him, Dias, Aké, and hopefully Stones. Since Dias came in and Laporte recovered from coronavirus so many of our defensive issues have lessened massively from last season, or even the start of this season. Yet we still have Mendy injured, we're still using makeshift LBs, we're still not using Fernandinho in midfield etc. But for the first time in ages we look strong at the back. Like really good too. We're barely conceding chances or goals consistently for what feels like forever.
Kind of like how we've had a few issues attacking this season due to having no forwards, we struggled last season due to having no (reliable) CBs.