Pieters arrived in the summer from PSV Eindhoven.
"I can only be happy with the form I have shown," the 25 year-old Dutchman told The Sentinel, "but I am still thinking I can do better.
"At the beginning, of course, I have to get used to the play of the Premier League. Also, after one-and-a-half years of injury I came back to play again.
"I felt good in pre-season, but the league is always different. Now I feel better and better.
"When I came back from injury it is a big step up to come back, and especially making the step up to the Premier League. But yes, I do feel there is still more to come."
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