Everton parted ways with Koeman after they dropped into the relegation zone following a 5-2 home defeat by Arsenal last month - a game in which Niasse scored his third league goal of the season.
"I feel sorry for him (Koeman), what happened to him when the club got rid of him," Niasse told British media ahead of Saturday's trip to bottom side Crystal Palace.
Niasse, 27, nearly joined Palace on transfer deadline day but chose to fight for his place at Everton.
"Honestly, they were ready to let me go. I was ready also to go because I knew that it would be a difficult season if I stayed here but at the end I didn't go," he said.
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(Reporting by Shrivathsa Sridhar in Bengaluru, editing by Ed Osmond)
