The forward joined Everton in January 2016 but within weeks of Koeman’s arrival from Southampton Fc that summer, Niasse was told he had no future at the club, despite costing £13.5m from Lokomotiv Moscow.
Niasse was left to rot in the under-23s before being farmed out on loan to Hull City last season and was on the verge of a summer move to Crystal Palace FC until it fell through on deadline day in September.
However, Niasse has since forced his way back into contention under caretaker boss David Unsworth and is currently Everton’s top scorer this season with six goals.
“When Koeman arrived I was a real good target to destroy,” Niasse told BBC Sport. “He tried, but I was strong enough to stay. I worked so hard to come to the Premier League so I had to stay in the Premier League.
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“For him (Koeman), he worked so hard to let me go. He tried to a do a lot of things that were not nice.
“Having taken my locker, my shirt number, he took me out of the dining room where we eat and took me to the Under-23s without no locker there. It was very difficult. You need a very strong mindset to deal with that because you don't have any respect at the club.
“Using the word 'slavery' is too much, but it was very difficult. That's all I can say.”