September to mid December - league winning form
Half the team out with injury from mid December through February - relegation form
Team comes back for the rest of the season - league winning form
I'm pretty certain our issues were 100% due to injury
That is last season in a nutshell, and I find it bizarre anyone can come to any other conclusion. The range of theories to explain it any other way are pointless when the reasons were staring us in the face from the team sheets each week. Some fans just need someone or something to blame - like suggesting Wijnaldum lost form (trying to run the midfield without Henderson or Fabinho next to him...?), or some forwards were misfiring (without the high press, less overlapping fullbacks, and less service from a different midfield), or that we didn't sign anyone (despite Thiago, Jota and Tsimikas), or that teams had 'worked us out' (despite not being able to at any other time over the last 3 years), or when we apparently spent a month 'sitting around before panic buying centre backs' (amazing how much some know about what happens behind the scenes).
If we'd had a fully fit squad I could understand some concern, but as you've pointed out - the correlation between results and injuries is clear as day, and I'm pretty sure we'll carry the momentum of those last 10 games into this season.
I still think a midfielder/striker would make the squad stronger, but if the focus has to be new contracts and financial stability for a while, then we still have a strong enough squad to challenge. Even Virgil and Joe returning makes a huge difference, as those two literally improve every area of our team as all great CB's do - they allow us to play a high line, allow the fullbacks to bomb forward, give Alisson more protection and allow him to be a proper sweeper keeper, allow our midfield to press more and be braver going forward to support the strikers, provide an aerial goal threat, and also help us beat the press with accurate passes to bypass the midfield. It's a cliche, but the returning players will feel like new signings and if we can win 8 and draw 2 of our last 10 without them, then imagine the improvement when they're back to full fitness. We'll also (hopefully) see Thiago and Jota get sustained runs in the team which can only be a good thing.
We've been brilliant for the majority of the last 3 seasons, and only lost form when we had the worst injury crisis in the club's history.