The Black Cats striker admits Di Canio's authoritarian approach to management left players deflated and afraid to step out of line.
The Italian was, however, to pay the price for a disappointing start to the season and has since been replaced by the more relaxed regime of Poyet.
He has helped to lift spirits within the camp, with Fletcher among those to have welcomed a much-needed change in the dugout.
Ahead of Sunday's Wear-Tyne derby clash with Newcastle United, he told The Sun: "The difference between the two managers is like night and day. Gus has given the place a buzz again and it needed it.
"His staff like us to tell them their opinion - it's not just their way or no way."
Fletcher says that was the case under Di Canio, with strict rules making life a misery for those on the playing staff at the Stadium of Light.
He added: "You had a little laugh and you felt like the head teacher was going to shout at you.
"Paolo would be the first to admit it as well, he didn't want you to smile in training.
"We would end up not even speaking because we were scared to say anything in case he cracks up at you. It got to the point where you didn't want to talk to each other because you just couldn't be bothered with the hassle.
"The boys would come in, get a shower and leave because they just wanted to get away. It wasn't a nice place to be.
"He could have become a Sunderland legend because the fans loved him. But the players never did and it is the players that need to like him."