It must be because of the horrible condition of the team first of all. Besides he hasn't been able to play like he was doing at Barcelona. And now there are new alternatives in the attacking line as well. There are Felix and Fofana as well with the hope of increasing the productivity in scoring goals. Nkunku is another player who is expected to join by this summer. Aubameyang will need to work much harder to earn his place in the starting eleven. But I don't think he can be as productive as his Barcelona performance.
The source: https://as.com/futbol/primera/aubameyang-podria-volver-a-espana-n/
I know that the arrival of Felix will disorganise the standing order of Chelsea attack which is not also strong before now. Wait till Sterling is back and Nkunku begins to appear for Chelsea.
By then the Chelsea attacking line will be like Sterling Nkunku, Felix.
But for now Aubameyang should relax because Felix is on loan and going back to Barcelona he will not find playing time.