When City initially signed De Bruyne from Wolfsburg for £55 million in 2015, there was plenty of scepticism over whether he could cut it in the Premier League.
He'd previously flopped at Chelsea, and some pundits pointed out that the Bundesliga was a far easier environment to perform in - most notably Arsenal great Paul Merson.
"Manchester City paying so much money for Kevin De Bruyne is an absolute joke." Merson said in his column for the Daily Star. "Okay, he's had a good season in Germany. But there are only a few decent clubs there really, aren't there? It's not the Premier League."
Oh how he was made to eat his words. De Bruyne helped City win the League Cup and reach the Champions League semi-finals for the first time ever in his first season at the Etihad, recording nine goals and 27 assists across all competitions.
City only finished fourth in the league, though, and Manuel Pellegrini was sacked to make way for Guardiola.
De Bruyne had already silenced his doubters, but few could have predicted just how good he would become under the Spaniard's stewardship.
After seeing him shine during a 4-0 victory against Bournemouth in September 2016, the former Barcelona boss said: "When we talk about [Lionel] Messi, maybe he can sit alone at the table, with no-one else allowed. But at the table beside, Kevin can sit there."
Guardiola was unable to deliver a trophy in his first season in Manchester, but he managed to figure out a winning formula. City broke the record for the most points in a single season (100) on their way to their third title in 2017-18, with De Bruyne emerging as the most important player in the team ahead of Silva and Sergio Aguero.
A domestic treble followed the following season, which De Bruyne missed the majority of due to injury, but he spoke candidly about Guardiola's influence on his career after recovering in time to help City down the finishing stretch.
“Pep and I share a similar mentality,” De Bruyne said in the Players’ Tribune. "He is not just interested in winning. He wants perfection.
“The first meeting I ever had with Pep, he said, ‘Kevin, listen. You can be — easily — a top-five player in the world. Top five. Easily.’ I was shocked. But when Pep said it with so much belief, it changed my whole mentality. It was kind of genius, I think. Because I felt like I had to prove him right, instead of prove him wrong.”
Since then, De Bruyne has added two more Premier League winners' medals to his CV while becoming one of only three players to win the PFA Player of the Year award in consecutive seasons - alongside Henry and Cristiano Ronaldo.
De Bruyne was a very good player when he arrived at City, but Guardiola unlocked his full potential and turned him into one of the all-time greats.
To achieve that, the City boss has had to be hard on him at times, including earlier this season. Guardiola insisted De Bruyne wasn't at his "top level" in October, and challenged him to do the "simple things" better after dropping him from his line up in March.
Some players would have lost confidence or downed tools completely, but De Bruyne is made of sterner stuff, and has since proven he deserves to be the first name on the teamsheet.
City are reaping the rewards at the business end of the campaign, with the ultimate treble on the cards if De Bruyne can keep reaching his maximum.