The Costa Rica star has yet to live up to his price tag after arriving in August from FC Twente for £10m.
But he completed his first full game for the club in Monday night's win over Liverpool and now wants more playing time.
Ruiz, 26, said: "It felt really good - especially as it was my first 90 minutes for the team and we won.
"I need minutes in matches - with those comes confidence.
"I'll continue working hard. I know I can do much better than I am now, but I am feeling happy with the job I am doing."