Southgate opted to rest eight players for England's final Group G fixture, with a 1-0 defeat to Adnan Januzaj 's stunning strike seeing the Three Lions finish second in the pool.
Colombia await in Moscow on Tuesday, with England having successfully avoided a potential quarter-final against favourites Brazil, instead lining up a possible last-eight meeting with either Sweden or Switzerland.
That has some England fans plotting their route to the semi-finals even though the team have not won a knockout game at a major tournament since the 2006 World Cup, most recently losing to Iceland at Euro 2016.
And Southgate, who has faced criticism for his weakened team against Belgium, is unconcerned about the pressure he is under at Russia 2018 following his team selection, with Group G winners Belgium earning a tie against Japan.
"I think whoever we played [in the last 16] we believe we're capable of beating [them]," said Southgate. "And whoever we play, in a one-off game, is capable of beating us.
"But we had to make a decision we felt gave us the best opportunity preparing the whole squad for that game, and that's what we tried to do.
"Of course, if we don't win, I understand people would say whatever about the Belgium match as well. But, for me, that was the right thing to do and you've got to take decisions you believe are the right ones.
"Maybe I have put more pressure on myself, maybe I haven't. That is the least of my concerns. The most important thing for me is that the players are in the best physical condition for the game on Tuesday.
"I don't think, mentally, we lose anything because they know we've made changes and they know Belgium weren't their full team as well.
"So, we're into big matches where margins will be fine and judgment on me will be extremely harsh. That's why we're here. We want to be in those games. To be trying to win a first knockout game for 12 years is exciting. We've got the chance to be the team that changes that.
"It's one of the reasons why we did what we did [against Belgium], to give ourselves the best possible opportunity of doing that. I wasn't so comfortable with the love-in in the build-up to Belgium, to be honest, so it's nice that there's a little bit of an edge back."
Harry Kane was an unused substitute against Belgium, but the England captain remains clear of the field in the race for the Golden Boot after scoring five goals in wins against Tunisia and Panama.
"I felt the need to sit with him but he was brilliant, absolutely brilliant. He totally understood, 100 per cent team first," Southgate added.
"He said: 'Look, I know everyone says I want the Golden Boot. Of course it's something I want to do. But the main thing is getting the team through the first knockout.'
"He was excellent on that, showed real leadership and understanding of the big picture."