The Cottagers paid out a reported £10.6million to bring in the Costa Rica international on deadline day after being convinced by his performances in the Eredivisie that he could be a great success.
But Fulham have yet to see a return on that investment, with Ruiz failing to score in his first month at the club and not yet managing to establish himself in Martin Jol's starting XI.
The 26-year-old concedes that his confidence has been hit by how different the demands of English football are compared to what he was used to in Holland.
"I've not had a comfortable time so far," said Ruiz.
"When I played in Holland I used to have an extra second to think.
"That makes a big difference. Here I am placed under greater pressure and get less time to act on the pitch.
"You also get more one-on-one situations in Dutch football. Here you get two players breathing down your neck straight away."