Attack is the best form of defence.
Improving the attack will have a bigger impact on Chelsea than people think. Against Liverpool, Chelsea could not hold the ball up the other end of the pitch until Pulisic came on. With better attackers, Chelsea will be able to pin their opposition back in their own half on a more consistent basis and lose possession less in such a position. This, in turn, will reduce the number of times the ball is down their end and consequently reduce the number of goals conceded.
On top of this, they should score more goals. This will demoralise the opposition (not all the time, but sometimes), which will put them on the backfoot. Again, this will help to reduce goal concession. If Werner, Ziyech, and Havertz (if they sign him) fulfil their potential, then it won't just increase goals scored but also the number conceded will reduce.
They have scored nowhere near the number of goals they should have this season. They have been wasteful, and have failed to put games to bed when they are on top because of their lack of talent up front. Giroud has his uses, but stats wise he is not great; Willian's output is also similarly low; Abraham is massively inconsistent, and can be pretty ineffective at times.
The question is what do you prefer to improve first? Attack or defence? I think the attack is a priority. We saw what happened with us when we chose defence over the attack in the summer. I think if Fernandes was signed in the summer and Maguire in January, we would have more points now. Remember that our goal concession did not really go down until Fernandes came into the team. Why? Because he is an actual threat, unlike Lingard and co.