All eyes were on how Melbourne, who parted ways with Christian Petracca (Gold Coast), Clayton Oliver (GWS) and Steven May (retired) over the off-season, would stack up.
Van Rooyen, so often the face of Melbourne's dysfunctional attack under Simon Goodwin, was the chief beneficiary of a slicker, fast-moving midfield in Sunday's 18.12 (120) to 15.17 (107) win at the MCG.
"I feel like I've been talking a lot around what 'Demon football' is going to look like, but until you put it on show, and actually do it, they're pretty cheap, those words," King said.
"Today was a really challenging game, as all early-round games are, and it was quite chaotic. I don't think we sit here today and (say) it was by any means perfect, we were challenged a lot.
"But really proud of the players; resilience and composure and the pressure just to win big moments and to get the win is really special."
Van Rooyen, 22, had five goals by halftime while ex-Collingwood forward Brody Mihocek added three majors.
"I was shocked. I didn't know he kicked six. I just saw his work ethic, his work rate, his energy - and that's what his whole summer's been," King said.
"So I'm really rapt for him."
Melbourne condemned Ross Lyon's Saints and their big-name recruits Tom De Koning, Jack Silvagni, Sam Flanders and Liam Ryan to an 0-2 start.
Goodwin's papers were stamped after Nasiah Wanganeen Milera's matchwinning masterclass late last year.
King sent Koltyn Tholstrup to the St Kilda maestro, who had been shut down by Magpie Harry Perryman in opening round.
Wanganeen-Milera finished with three majors and 18 disposals, but couldn't repeat his heroics.
King described Tholstrup's tagging job as "pretty special" while Lyon said Wanganeen-Milera "fought" and he would take three goals as decent output.
Melbourne debutant Latrelle Pickett kicked a magnificent first goal - weaving between St Kilda defenders, taking three bounces, then bouncing through a terrific snap.
The teenager was swarmed by teammates, including cousin Kysaiah Pickett, who was solid after spending part of the week in Darwin for personal reasons.
"Those two do some pretty things that are unscripted, and some magical things with the footy," King said.
Magnificent Melbourne skipper Max Gawn went toe-to-toe with Tom De Koning and completely outpointed his younger opponent with the game on the line in the final term.
Gawn played 91 per cent of the game and went one-out in the nail-biting final term - when he kicked a goal, set up another and had 10 disposals, three marks and 11 hitouts.
The 34-year-old finished the match with 22 possessions, 34 hitouts and a goal, while 26-year-old De Koning had 14 touches and 20 hitouts.
"For his age, just the way he grinds out games and uses the experience and the impact he had was huge in the outcome of the game," King said.
Former St Kilda skipper Jack Steele, unwanted last year by the Saints, had 20 disposals, five clearances and a goal.
Max Hall (four goals, 25 disposals) was St Kilda's best, with Marcus Windhager (32 disposals) and Callum Wilkie (27 disposals).
It was a back-and-forth affair until Kysaiah Pickett put Melbourne in front by five points - the 12th and final lead change - just before three-quarter time.
The Demons never gave back the lead.
"The reality is we could be 2-0, maybe, 1-1, but 0-2, regardless of what that result was ... we've got some issues to deal with and some things we're doing really well," Lyon said.
"As much as it grinds on you and they're euphoric and there's no in between, we certainly feel we've given great effort."
Lyon was unconcerned by injuries to Mattaes Phillipou (corked quad) and Liam Ryan (corked shoulder).
Melbourne travel to face Fremantle on Saturday and St Kilda go to GWS.
