There is a clause in the UEFA rulebook that offers a potential, albeit frustrating, escape route. The regulations add: "Exceptionally however, in such a case, a team qualified to play the second leg of these rounds may decide, by informing the UEFA administration before the draw, to reverse the tie and play the first leg at home instead of playing the second leg in an alternative venue."
This leaves Chelsea with a difficult decision. They could fight to play at Stamford Bridge on a Thursday night - an option described as "unlikely" by senior figures, according to The Sun, due to clashes with the Europa League and TV scheduling - or they can voluntarily give up their hard-won advantage.
Reversing the tie would allow the match to take place at Stamford Bridge, but it would mean playing the first leg at home and the decisive second leg away. It would essentially punish Rosenior’s side for qualifying alongside their local rivals. A UEFA spokesman confirmed that "discussions on scheduling are ongoing with local authorities," but as it stands, Chelsea are victims of their own - and London’s - success.