One of the things I love is how people ignore when information comes out. People keep saying the rot started with Ange. He has explicitly said that he knew he was gone by January, so had a choice to make. Either try and play on all fronts and not win anything, or win Europa. There is no point in pushing in the league when the best you can achieve is low mid table. So by forgoing the league and winning Europa, he secured them Champs League football, and a trophy.
To say he started the rot is unfair. The signings he wanted and what they got are completely different. Had he signed Semenyo, Guehi, Mbeumo and Neto, they'd be a far different team than having Solanke lead the line.