De Zerbi is suffering after over achieving last season. Since then he's lost Caicedo and Mac Allister, March and Mitoma have been out for most of the season, Estupinan has also been out for a significant chunk. Ferguson has had a typical season of a youngster breaking onto the scene and struggled a bit this year, when not injured.
We're not a United or a Liverpool with a huge bench and losing these players has had a massive impact on us and our form has definitely dipped since Christmas, but for Brighton to be a top ten club is a massive achievement and for him to do it with the players available to him is impressive.
There's a lot of recency bias with De Zerbi and Brighton, in reality it's only the last 3 seasons we've been a half decent premier league team. I'm quite happy to read a lot of posters writing him off, it means he'll stay with us for longer, but he's done a very good job with the squad available to him.