De Boer says losing Zaha to injury at the start of the season was a big influence on his sacking.
He told Sport Express: “I lost my best player in Zaha in the very first week, so his injury was our main problem - he is a real Messi.
“I tried to change the system but I failed. After the sacking I was very angry.
"Players were also upset, but that's the way it goes."