Hughes says a left-back was his priority after assessing the squad, which is why he signed both Pieters and 21-year old Marc Muniesa, who has arrived on a free transfer from Barcelona.
He explained: "Looking at the squad it was evident to me there was a lack of naturally left-footed players.
"We needed to address that, and there were good opportunities to bring in the right quality as well. Erik is a bit older and a little more experienced than Marc, but both have played at a good level.
"Erik has played at a top club in the Dutch league so technically, as you would expect, he is very good.
"He is a player I think will adjust very quickly to the Premier League."