21-year old Slager netted in the U-21's 3-1 win over Huddersfield Town a which he showed some ability to impress the management team.
Slager had signed for Coventry City in January on an 18 month deal but had his contract cancelled mutually after seven weeks as he wanted to pursue his devlopment back in the Netherlands.
The fomer RKC Waalwijk striker had a trail at Newcastle United in January 2013 and had been touted as being a big talent.