Al Hilal has signed Kalidou Koulibaly from Chelsea. Leaf fall continues at Chelsea. They will be missing all their key players. They lost stars like Kante, Koulibaly, Havertz, Kovacic. It is very difficult to replace them. What will be the new transfers, will these players be replaced? This is the main question.
It won't be difficult for Chelsea to find a better substitute for the guys you highlighted, in my opinion. If you look at Chelsea's new starting lineup, you'll see that the players you listed have already been replaced out for the matches of the upcoming season, and the new players are much better than the ones that were sold. Possible line up for chelsea next season.
When a player leaves a club due to a sale, a quality replacement is brought in right away. Players are sold and replaced before each new season during the transfer windows. That's football business for you(sold and bought better ones)