Defoe, 29, is considered by many pundits to be the “fourth choice” forward in Roy Hodgson’s squad for Poland and Ukraine.
But if England need a late leveller in a crunch game then the new boss is most likely to turn to the Tottenham man.
Defoe said: “I always try my best in every game.
“At any tournament you need players who will start the games but you also need those that can come on and make an impact — especially if a big match goes to extra time.
“You get to semi-finals and, hopefully, finals and then fatigue kicks in.
“When you sit on the substitutes’ bench you have got to just stay focused. You might get a chance to win the game later, you never know.
“So it’s very important to keep sharp, watch the game at all times and try to make an impact when you come on. I believe I can do that for the team.”