But Bert van Marwijk is much more than a former boss to Coumans.
He has been a mentor, colleague and friend for more than three decades, having first encountered Van Marwijk as a young player before eventually becoming his trusted assistant at Hamburg SV, Saudi Arabia, Australia and the United Arab Emirates.
Coumans' career has since taken him around the world, including head-coaching spells in Saudi Arabia, but he still points to those years alongside Van Marwijk as one of the most formative periods of his football life.
Speaking exclusively to Football Presse, Coumans revealed just how far back the relationship goes.
"I knew Bert van Marwijk already a long time.
"He was my youth coach, he was my coach when I started playing in the first team of my club in the village in the third division.
"So I've known him already since 1985 or something like this."
It was a relationship that continued after Van Marwijk moved into professional football.
"When he went to his first professional club, he brought me in there. Then he went to Feyenoord Rotterdam, and we didn't work together for 14 years."
Their paths eventually crossed again in a way that would change Coumans' own career. After earning his professional coaching licence, Coumans received an unexpected call from his former coach.
"After these 14 years, he suddenly called me when I got my pro licence to go with him to Hamburg."
It was the beginning of a remarkable eight-year association.
Coumans joined Van Marwijk's staff at Hamburg in 2013, before following him into international football with Saudi Arabia, Australia and later the UAE. His career record reflects just how long the partnership lasted.
For Coumans, the value was not simply being alongside a successful manager. It was having a privileged view of how a top coach thinks.
"From there on, I was eight years with him, and it was a very good experience for me just to know the way he thinks.
"You know, he shows it on the pitch, the things he says to the players, the decisions he makes and why, all these kind of things."
That access to the thinking behind the decisions was invaluable.
"It's nice to know from a top coach what's going on in his mind."
Coumans also learned something that can be difficult for an assistant to find at the highest level: the freedom to disagree.
"The best part is for me, he trusted me in my opinion.
"So I was always free to give my opinion and he was always ready to listen to what I had to say."
That trust became particularly valuable as the pair moved between very different football cultures.
At Hamburg, Coumans experienced the intensity of German football and the pressure surrounding one of the country's biggest clubs. Van Marwijk's spell ultimately ended after seven consecutive Bundesliga defeats in early 2014, with both manager and assistant leaving their positions.
But the difficult moments were part of the education.
Coumans subsequently accompanied Van Marwijk to Saudi Arabia, where they helped guide the national team to the 2018 World Cup. The Dutchman's staff included Coumans as the Kingdom began a period of significant football development.
The experience was followed by another major challenge in Australia, with Van Marwijk appointed Socceroos coach for the 2018 World Cup and Coumans again part of his staff.
For Coumans, the constant was the opportunity to observe how Van Marwijk adapted his ideas to different players and football cultures. It also reinforced one of the central lessons he took into his own career: a coach needs a clear philosophy but must understand the players in front of him.
"To know the way he thinks, the things he says to the players, the decisions he made and why — all these kind of things."
That lesson became increasingly important when Coumans eventually stepped out from behind Van Marwijk and became a head coach himself. His first senior head-coaching opportunity in Saudi Arabia came at Al-Hazem in 2022, before spells with Abha and Al-Adalah.
Yet even after establishing himself in his own right, Coumans has never lost sight of what he learned from the man who gave him his first major opportunity.
"Very, very good experience. I learned a lot from him."
There is also something revealing in the way Coumans describes their relationship. There is no attempt to present himself as merely the assistant who followed orders. Instead, he remembers a working relationship built around trust, discussion and understanding.
For an aspiring coach, being allowed inside the mind of an experienced manager can be more valuable than any coaching manual.
Coumans was able to watch Van Marwijk prepare teams, manage players, make difficult decisions and respond when plans went wrong. He saw the same coach operate in the Bundesliga and then adapt to international football on the other side of the world.
That experience helped shape Coumans' own philosophy as he eventually took charge of clubs himself.
And perhaps that is the greatest legacy of the partnership.
Van Marwijk gave Coumans an opportunity, but he also gave him something harder to quantify: a close-up education in how a top-level manager sees the game.
After working together from Hamburg to Saudi Arabia, Australia and the UAE, Coumans eventually stepped out on his own.
But the lessons from his old coach remain.