Thorgan Hazard played a key role for Borussia Dortmund as Lucien Favre's side dismantled rivals Schalke 4-0 on Saturday.
The Revierderby was the first fixture to be played since March due to the coronavirus pandemic, with the match played behind-closed-doors.
And Hazard was instrumental for the home side as he scored and created another for Erling Haaland.
Just like his brother Eden, Thorgan was a Chelsea player from 2012 to 2015, but it looks like the Blues might have let another good prospect slip through their fingers, along with Mohamed Salah and Kevin De Bruyne.
The Belgian didn't make a single appearance in his three years in west London, before departing for Borussia Monchengladbach.
Hazard enjoyed a successful four-year spell at Gladbach, scoring 46 goals in 182 appearances, before joining Borussia Dortmund last summer.
But during his spell at Gladbach, he had been tipped to return to west London with reports of the Blues having a buy-back clause inserted into the deal.
However, speaking in 2016, Gladback sporting director Max Eberl dismissed the idea of such a clause being in place.
"There is no clause," he told Express.de. "I do not want to comment on any contract content. But we also do not need to make any secrets about it.
"If we get such a top player from Chelsea, then Chelsea has, of course, always the opportunity, in whatever form, to think about this player."
Last year, Sport Witness translated a report by German outlet SportsBILD that claimed Chelsea had a right to match an offer if another club come in for Thorgan Hazard, rather than a straight buy-back clause.
"To be honest, even I don't know if there is one," Hazard previously said in an interview with Onze Mondial.
"My father [Thierry Hazard] deals with that now. Sometimes, I hear there is one. Here, I'm told there isn't. Frankly, I don't know.
"It's not that I don't care, but I don't think about it. I'm happy here. If Chelsea want to buy me back one day, I think they'll do it, that's not an issue. Actually, if there is or isn't one, it's the same, really.
"If Chelsea want me, they'll pay for my transfer and they'll take me back. It's just a detail."
Bild also stated that Chelsea's clause was only valid for the 2019 summer transfer window, which was of course before the Blues found about about their transfer ban, and the winger ended up joining Dortmund instead.
But in another universe, the talented Belgian could have followed in his brother's footsteps and established himself as a regular for Frank Lampard's side.
