Let's make this clear from the off - Chelsea keeper Petr Cech has been the most consistently excellent keeper in the Premier League since 2004.
The Czech Republic stopper has set Premier League records for clean sheets and goals conceded, and has won domestic and continental honours at Stamford Bridge.
But all good things must come to an end. Cech has been linked with a move to Barcelona, and it would be a good idea to sell him.
The 31-year-old is slap bang in the middle of the 'prime' window for top-class keepers - generally accepted to be somewhere between ages 29 and 34.
That means, if Chelsea want to receive any substantial fee, they'll have to sell him in the next few transfer windows.
Ordinarily, when you've got a world-class goalkeeper you aim to hold on to him for as long as possible.
But Chelsea find themselves in a uniquely privileged position. They have Petr Cech in his prime, and the world's best young keeper waiting in the wings to replace him.
Thibaut Courtois is into his third season at Atletico Madrid, and the 21-year-old has improved every year.
A Europa League trophy in his first term, and a Copa Del Rey in his second - including a stunning final performance against Jose Mourinho's Real Madrid - has established the Belgian as "one of the best talents in European football."
That quote is from Cech himself. But the Stamford Bridge side are approaching a zero-sum game. They will have to choose. It's either Cech or Courtois - the 21-year-old cannot be sent on loan indefinitely.
And if they want to maximise their chances of future success they need to back the Belgian.
For a start, they'll get a sizeable transfer fee from Barcelona for Cech. That can go towards improving the squad in other, weaker, areas - perhaps a striker, maybe a long-term replacement for Ashley Cole.
And while they will have lost a superb goalkeeper, their squad as a unit will have only got stronger. Courtois can be the next Cech, making the same kind of impact over the next 10 years. The only problem is the current one is blocking his path.
Just consider the flip side. If Cech stays, Courtois will more than likely leave, and the Blues will have lost one of the best young players in the world because they wanted four (five maximum) more years of Cech.
That doesn't make sense if your thinking long-term - 5 more years of Cech or 10/15 of Courtois? And Chelsea should be planning head - with Eden Hazard, Oscar, Juan Mata, Romelu Lukaku, Marco van Ginkel and the like their future looks good.
Add Courtois to that mix, and you've got the foundation for success, a better framework to work within than any other Premier League team.
It's a risk yes, you can't guarantee Courtois will live up to the hype. But every indication from La Liga suggests he will, and you took a chance on a 22-year-old Cech back in 2004 and look how that paid off. It's worth it.
It might be hard to let such a key part of past glory leave, but it makes sense. It will hurt initially, but the alternative for Chelsea would be much more painful.