Defoe ended a three-and-a-half-year absence from the global stage on Sunday, opening the scoring in England's 2-0 win over Lithuania at Wembley.
Defoe, 34, who was making his first England appearance since November 2013, hit the target midway through the second half for his 20th worldwide goal.
The 34-year-old marked his 56th cap with a typically astute strike after 21 minutes against Lithuania in their Group F World Cup qualifier, fully justifying his return after an absence of three-and-a-half years.
Manchester United's Wayne Rooney, Tottenham Hotspur forward Harry Kane and Liverpool hitman Daniel Sturridge all missed the qualifying win over Lithuania with fitness issues, leading Southgate to call on the evergreen Defoe to keep the Three Lions' route to Russian Federation 2018 on track.
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"I'm trying to turn vegan, which is a amusing one because when I go to my mum's house she's got every meat you can imagine out on the table", said Defoe.
"I think they are a better team than maybe they gave themselves credit for".
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Three policemen were among the dead while the rest were civilians, and a number of police were also wounded, the officials said. The offensive is part of a wider campaign launched last October to retake the entire city, which Daesh overran in mid-2014.
It means he has now scored in 16 consecutive Premier league seasons, and while it remains questionable as to whether Defoe's efforts will keep Sunderland in the division this year, the forward is as sharp now as he has ever been, scoring the second most goals by an English player, behind Kane (19). "I guess the ideal world for all our players is they are playing regularly at a high level", said Southgate.
"I'll not eulogise over the performance but it's job done".
In all, Defoe has netted 36 times in 84 games for Sunderland, with 14 of those goals coming this season, and he is now one of the top goalscorers in the Premier League for 2016/17. "I'm trying to turn vegan, which is a amusing one because when I go to my mum's she's got every meat you can imagine on the table".
"It's nice to score but just to win the game was important and we did that", he told the press.
"I don't know if we can have a distinct pecking order because players that are playing well deserve the opportunity". We've got really good competition for places.
"We will take a lot of positives from this loss, to see what targets can be set because what we witnessed today in the first half was unbelievable how skilful all those attacking players are and what amount of pressure we were put under. There's a lot of things I do away from training and away from matchday that help me perform, so I'll just keep doing that". We've selected him on form.
"If you don't feel that internal 100 per cent passion for playing for England, then I'm not sure it's for me to sell that to you". As players you appreciate that. I can't constrain myself on selection entirely but I know what I want to have. "His performances this season and his goalscoring form have counted for him".