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.