The Johns family returned to Cessnock Sportsground on Saturday but time spent out in the middle for the Knights against the Goannas was less than anticipated.
Jack, son and nephew of Knights' legends Matthew and Andrew respectively, didn't come back on the field after undergoing a HIA just 15 minutes into the pre-season trial.
He had started at lock, having signed a development deal with the club after debuting with South Sydney in 2020.
The Knights, who had just a few players with NRL experience including Johns, defeated last year's Newcastle Rugby League premiers 20-4.
New recruit Dom Young scored the opening try and set up the second for Jacob Kiraz to put the Knights ahead 8-0 after 33 minutes.
Scott Haynes replied for the Goannas to reduce the margin to four at the main break.
Knights halfback Kobe Davis made it 12-4 in the 52nd minute before Dylan Lucas and Liam Wilkinson both crossed during the last quarter.
In the earlier games, it was mixed results for the Knights against the Panthers with a 30-24 win in SG Ball and 40-6 loss in Harold Matthews.
Next up the Knights top squad travel to Albury to meet reigning champions Melbourne this weekend.