I personally didn't think Tuchel was a bad coach. He has tried his best to make the team strong. To strengthen the bonding between team players. But for some reason, he could not succeed.
Yes, Tuchel is a great coach but he is not the perfect coach to keep his team in good form every season. Last season he looked to be on track, but earlier this season things changed even though some thought Tuchel was worth keeping as coach. Other than that, I don't think Chelsea's dip in form is Tuchel's fault, he just doesn't have a good squad to hope to have as good as expected.
Players like Raheem Sterling and Emerick did not adapt well to the team. If Sterling had been able to play better, Chelsea's performance would have been stronger. Chelsea's main weakness this season is their defense. If the defenders had played well, they would have been a little higher on the points table. Looking forward to seeing what steps new pitcher Graham Potter takes to strengthen the team.
Potter has to propose various changes to the current Chelsea squad, that is an important task to think about as well. He may be able to maximize the existing players, but some additions are mandatory.