The Cottagers signed the Costa Rican international last summer for a fee believed to be in excess of £10m, but Ruiz failed to make a regular impact at Craven Cottage.
Ruiz told the club's official website: "It would be nice to score a few more goals as the new season progresses and that is something I know I am capable of doing.
"I think there is more to come. I know that the manager has talked about me making a greater contribution and he has faith that I can."
The 26-year-old scored two goals in 27 appearances for Fulham last season.