The 23-year old Dutch striker joined the Magpies on loan from Germany's Borussia Monchengladbach in January but has failed to hit the back of the net in eight Premier League appearances since.
The Swiss-born de Jong rose to prominence when scoring goals for fun at Dutch club FC Twente between 2009 and 2012 but has struggled since and Pardew has implored him to prove himself at St James' Park in order to win a deal with the Toon.
"It's very difficult for a player who has been at a club and not been used after a big money move," said Pardew.
"He was a big player, a big personality who scored more goals in Holland than (Wilfried) Bony for example.
"He is finding his feet and finding his confidence. I think it's important his all-round game improves from where he is right now.
"I think he has had some good displays for us, but I think there's more to come. I hope between now and the end of the year that he shows that.
"But, at the end of this period he will have had three-and-a-half months with us and he needs to be pretty close to his best if we are going to sign him."
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