Defoe came on for the final 24 minutes of England's 2-0 friendly defeat to Chile on Friday night to earn his 55th cap.
"Getting into a World Cup squad is based on merit and you have to play for your club," said Defoe.
"As a forward, if you are not getting the opportunities to play it will be difficult.
"It's important that everyone goes back to their clubs and plays games to try to impress the manager.
"It's a long way to go before the World Cup. There is a lot of football to be played and we will see what happens. You never know, I might play every game from now on."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball