Fletcher insists the difference since new boss Gus Poyet took over has been like "night and day''.
Fletcher, 26, told the Daily Star Sunday: "Martin O'Neill was a massive influence. When I knew Sunderland wanted me, it was a massive club and I wanted to come here.
"But part of it was playing under Martin. I was disappointed when I got injured, then a week or so later he got the sack.
"It was hard. I was away in Dubai. He'd given me two weeks off and I was on the beach and got a text saying he'd been sacked.
"The rest of that season I was injured so I didn't really see the new manager, except in the corridor. In pre-season, I wasn't involved as I was still coming back from the injury and then, when I did come back, it was a surprise.
"I was shocked to see how intense it was - not the training, it was hard training but anyone can do hard training - it was just the atmosphere, the intensity of it all. It was just a weird atmosphere.
"You had a little laugh and you had the feeling your head teacher was going to tell you off.
"He said he didn't want us to smile and laugh in training - he said that himself, he can't deny that.
"But every other club I've been at, it's been a big part of things, having that togetherness. To be scared to say anything in case he had a crack at you, it's not nice, is it?
"We weren't scared to talk to each other but it got to the point where you just didn't want to.
"The boys were just coming in, doing their job and leaving. They just wanted to leave because it wasn't a nice place to be."
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