Well looking at Richarlison's contribution at face value, I think he's averaged something like 10-11 goals in the league a season since he's been here. Some will say he's a lot more valuable than just his goals because of his work rate and the passion he shows playing for us (which is unarguable), however I'd argue that his all round game like his dribbling and general wing play is quite poor.
When it comes to replacing him, I think we will be able to get someone in with significantly better ability but lower work rate fairly easily, it's just a question of whether they can score roughly as many goals as he can. 10 a season isn't anything spectacular like, but we'll soon see I guess.