Chelsea really didn't have such serious problems under the last owner, but I doubt he ran the club so well that things would start to crumble to such proportions after he left. Who knows, maybe he can still make a comeback. I hope in the new season to see a less chipper Chelsea that can fight for the top 5 in the table
Your wish seems unreal after Chelsea's horrific performance against Manchester City's young squad. I'd rather take 10th as the possible for Chelsea to finish in the current season. Chelsea played poorly in the pre-season games. The match against City showed Chelsea's weak defense and poor form. These are their main problems.
They never really replaced Thiago Silva and Rüdiger. Signing an elite CB needs to be their priority this window; none of the ones they have are good enough.
Also, Chelsea conceded four goals against Manchester City. This was due to their failure to transition back to defense after losing the ball in the buildup. This is a drawback of the inverted fullback system.
I don't think preseason games mean anything, especially for me as a fan it's basically a match for fun. Preseason games are a chance for coaches to try something and see if it works. If it doesn't work, then you have to change something, so it's quite possible that Chelsea will change something and play differently by the start of the season. There's not much time left and we'll see the results soon...