The Citizens knew they needed a win after Manchester United's 4-0 win over Crystal Palace earlier in the day, and got it with a goal from their Belgian playmaker in the 67th minute.

It was a damaging blow to the Blues on what was a fairly comfortable evening for Pep Guardiola's men, meaning Chelsea fell six points short of the league leaders.

And Antonio Conte was left to rue the fact that the opposing player who landed the decisive blow was once wearing the blue colours of the Stamford Bridge outfit.

“I don’t know, honestly, what happened in the past,” Conte said per the Guardian. “But for sure we are talking about a top player, the complete player. He’s good – technical, fast, and he works hard for his team. He is the complete player.”

De Bruyne quit Chelsea for Bundesliga outfit Wolfsburg in 2014 - during Jose Mourinho's time in charge - but is now enjoying a fruitful career back in the Premier League at the Etihad.

The 26-year-old's goal against his former employers on Saturday was his first of the season while having already racked up three assists from seven games this campaign.

And while no doubt the home supporters were bitterly ruing why the club ever allowed the Belgium international to leave, Chelsea legend Frank Lampard has supported the club's decision to sell.

Lampard: Chelsea had to let De Bruyne go

"It was a difficult time for the club, you had (Eden) Hazard, Oscar, and Willian had just signed. He wasn’t as good as he is now," Lampard told BT Sport, per the Metro.

"Of course, it’s easy to look and say 'why did you let him go?', but he’s gone away and made himself a world-class player."

Chelsea's lack of faith in giving De Bruyne game time may have ultimately cost them, but it's no surprise given their poor track record of bringing through even their own academy players.

And although Lampard accepts there was no way into the Chelsea team for De Bruyne at that time, the 39-year-old has hailed the City star's attitude to prove the Blues wrong.

Chelsea legend praises Belgian's 'ambition'

"You always wonder about development. He had good pace, good passing, good dribbling, but at that point he couldn’t get into our team," Lampard added.

"One thing he had was ambition, he wanted to go away and prove he was a player."

De Bruyne is certain to be an instrumental figure for City this season and could be the difference in Pep Guardiola landing his first Premier League trophy.

But Chelsea won't be turning to look back at Mourinho's decision to sell De Bruyne in 2014, however, rather more their missed opportunities in the summer transfer window.