The Portuguese right-back, who moved to La Liga from Sporting Lisbon in 2012, enjoyed a successful first season with the Spanish outfit.
The 29-year-old's form has dipped in the new campaign, but he has vowed to improve in the second half of the season.
"Not every player is always in top form and I am not exception. I know I'm not in my best form, but we're working hard to show the quality befitting of a club like Valencia," he told reporters.
Valencia are currently ninth in La Liga, 15 points off the top four.