Fellaini's definitely not as good a CM as Herrera. The skillset for CM is very different from an AM, which is where Fellaini's best at. That said Herrera wasn't particularly great either yesterday, but our midfield was pretty outnumbered yesterday with Fabregas, Kante, Bakayoko and Hazard roaming all over the place vs our 2. Having Pogba back after the international break will help us out a lot.
I thought Fellaini did very well when he came on, allowing us to go route one to Fellaini meant Chelsea had to sit back and it afforded it a lot more space to our attackers. It may not be 'pretty' but he's a very useful option to have upfront, and I'm glad we have him. People do need to recognise though that his best plays happen near the opposition box, and not ours.