With the second-leg of their Champions League against Barcelona looming, Hiddink warned: "You cannot afford to make the slightest error.
"I was not happy because we were sloppy in some positions and that's what I didn't like despite a good win.
"It's not just one position, it was a sequence of errors mostly. If we want to play a little bit for ourselves when we are not fully concentrated, then we give away chances, like we did against Fulham, but, if we do that on a higher level, you will be exposed and punished.
"I was happy with the result but I don't shut my eyes to more moments in the game where we were sloppy."