Just 16 days after leaving on loan, Nicolas Jackson could come back to haunt Chelsea with Bayern Munich on Wednesday.
That is because -- contrary to Premier League rules -- on-loan players are eligible to face their parent clubs in UEFA competitions such as the Champions League.
While the likes of Oleksandr Zinchenko have already been prevented from playing familiar foes this season in England, UEFA's rules on the 'Integrity of Competitions' prevent clubs from exerting "any influence whatsoever over the players that another club may [or may not] field in a match."
It is a rule that was exploited by Atlético Madrid in 2014, when Thibaut Courtois starred against Chelsea in a Champions League semifinal, and by Monaco in 2004 when Fernando Morientes scored a goal in each leg of a quarterfinal to knock parent side Real Madrid out.
More recently, Kingsley Coman netted for Bayern Munich against Juventus in 2016, while Philippe Coutinho scored twice and assisted another in an 8-2 Bayern win against Barcelona in 2020.
The rule means Jackson, allowed to leave by Enzo Maresca despite an injury to new signing Liam Delap, is able to face Chelsea on Wednesday.
Jackson arrived at Bayern, sources have told ESPN, for a loan fee of €16.5 million ($19.5m), with the Bundesliga side having an obligation to sign the 24-year-old permanently for €65m. It is reportedly the most lucrative loan deal of all-time, although Bayern honorary president Uli Hoeneß has since claimed a full-time transfer will never be triggered.
Whether Jackson starts at the Allianz Arena on Wednesday is another question. The Senegal international made his debut off the bench in the 5-0 win over Hamburg on Saturday, and manager Vincent Kompany wouldn't disclose his starting XI when asked in a news conference on Tuesday.
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Maresca, meanwhile, admitted Jackson could give Bayern a competitive advantage.
"I don't have any kind of problem with Nico," the Chelsea boss said. "He's a good guy, a good professional who worked well with us. That's it.
"It's a small advantage because [Kompany] can ask Nico the way we work and the way we try to prepare games. He knows exactly the way we approach games."