Form and confidence dropped as a collective, but Ronaldo being here meant everyone had to change how they play, cover for Ronaldo being literally the most useless player in the top 5 leagues when it comes to defensive work (statistically, bottom 1%), doing not much else in the build up or off the ball movement... So he was just a black hole for any play. Rashford, Bruno, etc all can't play their normal games, team starts struggling, confidence plummets.
There were many other factors and not just Ronaldo, but it's no coincidence that no well run team wants anything to do with Ronaldo at this point, and that in back to back teams, they have struggled to get any success as a team with him in the team. He's 37. The time comes for everyone. But having him in the team is effectively playing with a red card until he gets the ball in the final 3rd and you hope he can score. And even there... He had a good third of the season where he was useless on the ball too.