Front rower Sims will get a start on the bench in place of Dean Young when Saints meet with Penrith in this weekend's Mother's Day clash.
The Gerringong teenager will become the ninth player to make his NRL debut for the Dragons this year.
Last week he played for the Steelers in the Jersey Flegg semi-final. The Steelers lost to the Sharks and Sims received a grade one striking charge. He avoided suspension after taking an early guilty plea.
Born in Sutherland, Ashton attended Kiama High, which also boasts ex-students, Rod Wishart and Shaun Timmins. Sims made his rugby league debut with Gerringong Lions and was an Australian schoolboys representative last year.
Sims said, “I thought Browny was geeing me up” when advised of the decision at training today.
“After he told me, he said to ring my mum on his phone... and he spoke to her - she was stoked,” he said.
Sims said he also spoke to former Dragons great and coaching co-ordinator Craig Young.
“He told me to do what I would usually do … so I'm going for a surf this afternoon,” Sims said. “He said not to think about it too much or you'll use up all your nervous energy and you'll feel flat on Sunday.”
In other news, Dragons captain Trent Barrett is expected to play on Sunday after doing rehab training today on an exercise bike. He will have a fitness test tomorrow.
Luke Bailey is expected to be out for two weeks after he was re-assessed by Dr Martin Raftery today. Bailey received physiotherapy treatment on his severe back strain.