It's remarkable that Penders has become a specialist with his feet given it used to be his weakness.
When Genk first scouted Penders from Sint-Jan Berchmans College, alongside his games for grassroots club Bregel Sport, his ability on the ball was his Achilles' heel.
"One of the teachers recommended him to me," Roex, who recently left Genk, added. "When I saw him, he had a great profile, he played well and he had a great mentality. But technically, he needed a lot of work.
"At that time, his feet were terrible because he was so tall. But he passed the trial for our under-14s when he was 13."
Roex says he has "never seen a player improve so quickly between the ages of 13 and 18" because of Penders' "unbelievable mentality" and his "inability to feel pressure" - qualities he insists remain evident today.
He added: "In the beginning he struggled a lot, but he worked very hard. We did footwork for at least 20 minutes in every session with our goalkeepers.
"He reads the game very well, which is essential because playing out from the back is not just about technique, but decision-making - understanding space, pressure and timing."
Nearly 20 years after helping develop the 6ft 7in Courtois along with fellow Genk goalkeeping coach Guy Martens, Roex said that Penders - who is the same height as Courtois - bears similarities to the former Chelsea goalkeeper, now a Real Madrid and Belgium great.
"Guy and I shared the same vision," Roex explained. "We believed football was changing and that goalkeepers needed to be comfortable with their feet. We implemented that philosophy throughout the academy, although there was initially some resistance from coaches.
"Part of our vision came from Ajax and Johan Cruyff's goalkeeping coach, Frans Hoek. He was a visionary who later worked with Barcelona and the Dutch national team.
"We were also early adopters of filming goalkeeping sessions, using tapes and simple editing software. We wanted to expose goalkeepers as young as 16 to the first-team environment.
"We produced Courtois and Penders, but also Koen Casteels, Martin van der Voordt at RB Leipzig, Kiaba Brughmans and Nordin Jackers, who is now at Club Brugge."
Roex believes Penders' calm demeanour comes from his close, stable family unit and having grown up in the small village of Maasmechelen just outside of Genk.
"We have produced a lot of great goalkeepers, but Mike is the one with the potential to reach the same level as Thibaut, his sense of calmness is gold in goalkeeping."
