Fernandes has shone since his January arrival from Sporting CP.
But he also says: "English football is different from the rest.
"Here teams come to Old Trafford and want to put pressure on, attack and take risks.
“The intensity of the game is very high. I was thinking fast in Portugal but here I have to think twice as fast."
Fernandes also admitted he struggled to believe how much United paid for him, with the potential outlay totalling £67.7m if all add-ons are achieved.
"Never were the values so high as now," said Fernandes. "I think it's an exaggeration.
“My transfer, the figure mentioned, the add-ons that could still be, the value that was paid for me was very high. Players that were bought at that value some time ago were Golden Ball winners."