The 30-year-old has scored five goals this season, including a hat-trick against Huddersfield.
"Honestly, I feel fantastic. Dr [Ramon] Cugat did amazing work with my knee and now I don't feel any sort of discomfort," he told the club's website.
"During the past few years, I felt pains at times and when last season came to an end, we decided it would be best to start treatment. And the results were very positive.
"I'm not sure if this is the best start [to] a season for me, but it's the best I've felt in years. It shows in my game. The challenge now is keeping up this level, and I'll be working non-stop to maintain it."