A rush of goals at the start of the season saw Defoe stake a strong claim to be used alongside Wayne Rooney, but Heskey kept his place for the recent games against Slovenia, Croatia and Ukraine.
With qualification assured, Capello used his final qualifier against Belarus to experiment but Defoe was injured, allowing Peter Crouch to score twice in a 3-0 win at Wembley.
Defoe's contributions as a substitute have been consistently impressive but the Tottenham striker is hoping to edge out Heskey in time for the World Cup and views Saturday's game in Doha as a real opportunity to press his claims.
"Do I need to re-establish myself? I think so. Peter Crouch came in the last time and played well, and he is a fantastic player," Defoe said. "It was frustrating to miss out through injury and when you get an opportunity, you have got to take it. You have to make an impact.
"I don't want to have the title of super-sub but when you don't start it is important you try and make an impression when you come off the bench. I haven't started a game for England in 2009 and if the first one was against Brazil it would be special because they are the side that everyone wants to play against."
If selected, Defoe's Tottenham team-mate, Tom Huddlestone, will win his first cap and the striker believes his colleague could have an outside chance of being one of the chosen 23 to travel to South Africa next season.
"Tom is a fantastic player and he deserves this call-up. He was in the squad last year against Trinidad and Tobago and did well in training and he is a natural footballer," Defoe said. "If he gets the opportunity, he will be ready. He could force his way into contention for the World Cup and he scored a fantastic goal at the weekend but ultimately it is up to the England manager."
One player guaranteed of his place is Wayne Rooney after the Manchester United scored nine goals in nine games during the qualifying campaign. Despite England's success in losing only one tie en route to qualification, Rooney is demanding constant improvement as the finals move ever closer.
"Of course we are in with a chance," Rooney said. "But there is a lot of hard work to do yet and there is a lot of improving that we have to do if we are going to win it. Our results have been better in qualification than they have in friendlies.
"We did beat Germany but Brazil are number one in the world and this is a massive game for us. We need to play some good football if we are going to win."