If we can be observant enough, Chelsea conceded enough goals from their central back and right back which indicates the incompetence of those players handling those positions. There was lack of communication between them at that defense especially when Thiago wasn't featured in the match, mistakes upon mistakes with same thing happening during this preseason. The one defender who has been top notch the whole season has been Cucurella and they owe him a lot because he delivered on his own part.
Chelsea should work with the defenders they have now and forget buying more, let them build the connection/communication among themselves instead of adding and then lack level of commitment, loosing the ball at critical points will only send the team down the table.
Defense is still a weak point that Chelsea must improve and this was seen not only during pre-season but also in the competition that took place last season. Chelsea did not prioritize these positions to be improved and instead they bought several players who were not quite right last season so there were many positions that did not work optimally when they played. Cucurella showed his best abilities in pre-season but he did not control the defensive position himself without the support of other players.
The coach must try to build good cooperation with the team and they must be able to communicate with each other. Next season the competition will be even tighter as almost most clubs have prepared the strength to compete for the title. If Chelsea returns to maximizing its existing strengths, it is feared that they will not be able to compete.