Rodri has been more important than many realise, he elevates the whole team, linking defence and attack, and destroying opposition's attacks.
His injury means other players have to work harder, which leads to injuries, which means the fit players are overplayed (Akanji, Gvardiol, Bernardo, Haaland), which leads to them losing form...
Of course, it's not only about losing Rodri. A sudden dip in form is perfectly natural after 4 great seasons. There's a reason why no other team in English history has won 4 in a row, and no other team in Europe has ever gone 26 games unbeaten. Those immense efforts have taken their toll. It's inevitable.
Why haven't we prepared for this moment a bit better? Because nobody knows when it will come, and because we owe respect to the players who have achieved the success. We can't tell them, say, Kevin or Bernardo, thanks but now please go. Also, some expected a crisis last season, and had we tried to replace players then, we might have failed to win 4 in a row.