This is how I would make use of our current squad. Ideally we would already have Stones and Varane at our disposal, they are here to improve the agility and speed of our defense.
The problem with our current squad is more of mentality and morale issue. After a disastrous beginning of the season and having a vulnerable defense most of the time, most seems to lose their appetite to play in a high level.
We were used to be unbeatable, bear the spirit of steel. Now our players seem to embrace the underdog title. None of the elite clubs are consistent this season to be fair, and the issue I see is all are without a dominant defense strength.
In the past years, Chelsea forged that Maginot Line under captain JT, Manchester United under Vidic and Ferdinand, Manchester City under Kompany, Arsenal under Sol Campbell (sorry IMO Liverpool is missing a little to qualify), ACM under Maldini and Nesta, Juve under Cannavaro, Barca under Pique and Puyol. They are the foundation of all these past champions.
Of course they were helped by effective DM like Makelele, Keane, Viera and so on.
Last season, the combination of JT, Cahill, Matic worked well. This season, the partnership is crippled by aging and fatigue.
So my focus would be rebuilding a dominant defense power, featuring the young Stones, Zouma, Varane, Azp. and led by the experienced veteran Cahill. This line should possess enough strength, speed, talent and passion to be the new almighty shield. Once we got a reliable defense, our talented attackers and Fabregas should be able to refocus on winning the game, rather than much distracted like this season.
Costa is our new Drogba/Aguero/Ibrahimovic. Oscar can be groomed to be our Modric/Iniesta. Hazard is our Ronaldo/Bale/Messi. Fabregas is our Xavi. We certainly have enough talent in our attacking force. And then Courtois will be our new Cech/Casillas/Buffon/De Gea for sure.
Under the same formation, if Isco is available in the next transfer window, I will get him to compete with Oscar. It's good to have Lukaku back, problem is he's not gonna sit on the bench for long yet it doesn't make sense to sacrifice Costa for him. So I don't see him coming back to Bridge anytime soon. Kenedy is just a temporary patch on the defense, but he is more of an attacker. We have yet to see Pato in action, however should he be able to recover his best form, he's going to be a killer inside forward. Cuadrado might still be a hidden gem, IMO it's still too early to write him off. And it's always good to have Pogba, but we have to be realistic considering we are not playing in the CL next season.
Side note: I am a long time Football Manager player, by no means I am a football professional. This formation is very effective in FM, whether it's going to work in real life is beyond me to discuss.