Chelsea lost 2 strong center-backs, Rudiger and Christensen. Their defense is decreasing so much, I am not really sure they can compete for the EPL title in the next season. I even think they won't do much in UCL competition with a big drop in their squad quality. Moreover, they seem to lose their main CF (Lukaku) because he feels uncomfortable and wants to leave. With having no strong defenders and weak attacking, Chelsea may be difficult to improve their performance and even very possible to experience the same thing as Man United.
Christensen wasn't that impactive this season he wasn't been used much. Our defenses consisted mainly of César Azpilicueta, Rudiger, Tiago Silver, and Alonso then Trevoh Chalobah and Christensen were rotated into the team whenever others weren't fit. Lukaku wasn't our main CF. He barely got game time and all this contributed to his departure plans. Kai Havertz stepped up and was the one leading our attack and the team was more productive when he's playing then Lukaku.
Kai Havertz was more impactive this season than Lukaku was and that's why they had similarly goal statistics although due to Havertz chemistry he had more assistant than Lukaku. Chelsea needs only two quality signings to get everything back on track.
Although Christensen did not contribute as much as Rudiger and Azpilicueta, Christensen's performance has been quite good this season.
So it's clear Chelsea have lost enough with Christensen's departure, moreover Rudiger and Azpilicueta will not strengthen Chelsea next season.
This means that with Chelsea losing many defenders, Chelsea's summer transfer market must first focus on bringing in new defenders.
So far Chelsea are rumored to be still trying to get Jules Kounde from Sevilla, and it is estimated that Sevilla will be willing to sell Kounde
to Chelsea for a transfer fee of 65 million euros. It looks like Chelsea will be negotiating again regarding the transfer fee desired by Sevilla.
Then regarding Lukaku's departure to return to Inter, it doesn't seem like it will affect Chelsea's performance too much. Since Lukaku proved
to be failing to adapt at Chelsea, as you said Kai Havertz is much more effective as a striker than Lukaku. But Tuchel is rumored to want to
strengthen Chelsea's front line as well, therefore it is rumored that Chelsea are eyeing Sterling and Gabriel Jesus from Man City.
Because the two players will certainly be sold by Man City, after it was confirmed Man City brought in Haaland and also Julian Alvarez.