Blues manager Jose Mourinho has since admitted that Courtois will be his first-choice keeper, and Keown believes that Cech is now left with no option but to quit Stamford Bridge.
"Petr Cech has already achieved greatness in a Chelsea shirt but by not picking him last week, Chelsea boss Jose Mourinho has left him no way back into the team," he wrote in his column for the Daily Mail. "Unless Jose has developed a hare-brained scheme where his goalkeepers can share duties, there is no choice for Cech. The door to the first team has been ruthlessly slammed in his face, so the only door available to him is the exit.
"Of course it's still early in the season and much depends on his character, but most top footballers have an element of selfishness. Your place in the team comes to define who you are and if you're not playing, it affects you as a person.
"Cech may not want to run away from this challenge and might relish the competition for his place. But if he is determined to play regularly, there is no other option — he has to leave."
Cech has spent the past 10 seasons at Chelsea since joining from Rennes in 2004.