Cummins' shock start for Australia in Johannesburg back in 2011 followed a similar script to the Agar phenomenon, and remains one of the best fast bowling debuts in Test cricket, despite it coming away from the hype of the Ashes.
At 18 years of age and with just three first-class matches under his belt, Cummins took seven wickets for the match and hit the winning runs in a nail-biting two-wicket win over South Africa that will go down as one of Australia's greatest.
However, Cummins found out quicker than most that no cricketer can stay with their head in the clouds forever.
Touted the future of the Australian attack, devastating injury problems have prevented him playing another Test since.
While 19-year-old Agar is preparing to fulfil a dream and play at Lord's, Cummins is in Zimbabwe on the comeback trail with Australia A.
Cummins has no doubt Agar has the maturity and balance in his life to handle his new-found stardom, but warned handling the inevitable downturns is the key.
"Maybe it's not an artificial high but I guess in careers, there's ups and downs. I think it's just about realising that," Cummins told AAP.
"Speaking to people who have been through it before, it's about trying to manage them.
"Realising the ups shouldn't be as high as they are. And the lows are never as bad as you think.
"One minute you can feel like you're on top of the world and, next minute, it can be forgotten pretty quickly.
"I don't think this is going to be forgotten but, if it's a career highlight, it's a great highlight to have for him."
Cummins is aware of the similarities between his debut and Agar's - both teenagers, both relative unknowns, not originally included in the respective Test squads and both with nerves of steel batting in the tail with pressure high.
"I suppose I knew what he was going through in some regards," Cummins says.
"I think he looked quite fearless and you're so happy and over the moon to be playing in a Test match that you just go out and soak it in.
"You know no matter what happens that no one can take it away from you. And that's something a lot of debutants feel. They just go out there and play without putting any pressure on themselves to perform."
Cummins is just 159 days Agar's senior and first came across the left-arm orthodox spinner in a NSW-Victoria under-17s match.
"We lost outright. I think Ash got me both innings," recalls Cummins.
They were teammates for the Australian under-19s and toured together again on the recent Australia A tour of the UK.
On the A camp, Agar roomed with Alex Doolan, and would stay up at night watching spin bowler clips on YouTube.
Cummins says it was surreal to watch Agar's magical 98 batting No.11 in the first innings when, just a few weeks ago, he was focused on simply getting back to Western Australia for a good pre-season leading into the Sheffield Shield.
"I was just back home in Sydney watching Ash go and it was just unbelievable," he said.
"Just how natural and how confident he was when he was batting was a bit of a breath of fresh air. I've seen a bit of him bat in the nets and he always looks like a proper batsman so I guess, while it was surprising, I think a lot of people who have played with him know he can bat really well."
The pair have exchanged text messages since, but it hasn't been entirely congratulatory.
Cummins says mates haven't let the chance go by to rib Agar over his new standing as sweetheart and heartthrob.
Marketing deals are sure to be waiting for Agar, who is also studying to be a lawyer, when he returns home.
But Cummins says the stardom won't change him.
"He's impossible not to like really," says Cummins, another genuine nice guy.
"The thing about him is he's got a really balanced life.
"He loves his cricket but, no matter what happens, he just looks at it as a sport.
"It isn't his whole life. He's got a lot of other stuff working for him too."