The forward, 19, has made two substitute appearances for the Eredivisie side this season and is a key part of their successful second-string team which is currently higher than Feyenoord, Ajax and PSV in the Dutch reserve league.
His contract runs until the summer of 2014, with the club having the option of extending it to another season.
But Waalwijk's technical manager Janus van Gelder says he is happy for the teenager to try his luck on Tyneside.
Van Gelder told Dutch newspaper Brabants Dagblad: "Slager is a big talent. He will get the opportunity from us to prove himself at Newcastle in the first week of January."
Source: Newcastle United Mad