Southgate raised eyebrows by naming Trippier at left-back for England's opening Euro 2020 match against Croatia on Sunday, with Luke Shaw on the bench and Champions League winner Ben Chilwell among three players to be left out of the squad.
It was only Trippier's fifth senior start on the left of defence, with all but one coming for England, and he put in a solid display in a 1-0 win, sealed by Raheem Sterling's goal.
With Scotland the visitors to Wembley on Friday, Southgate did not rule out Trippier continuing at left-back and says the Atletico Madrid player is a different animal to the one who left Spurs in summer 2019.
"What Kieran has, firstly, is hugely underestimated," Southgate said.
"I think people are still seeing the lad that was at Spurs a couple of years ago and not the defensive animal that has grown at Atletico Madrid, trusted by one of the best and most successful coaches in the world [Diego Simeone].
"He gets his distances right on cover, he’s good one-v-one to stop crosses, and his talking is really good. We had a back four, there was always going to have changes in it on Sunday and we felt that communication, that organisation was an important part.
"We’ll look at every opponent, we’ll look at our team for every game. I think we have to have flexibility.”
Jadon Sancho was also left out of the 23 to play Croatia at Wembley, along with Harry Maguire, who Southgate said is "progressing well" as he works his way back from ankle ligament damage.
"With Ben and Jadon, this was the downside of a squad of 26 – to have to have conversations with boys that don’t deserve to be left out," Southgate said.
"But we’ve got cover in certain positions and for the Croatia game we went a certain route with Trips at left-back.
“Everybody knows the competition for places in those attacking areas. So through no fault of their own we have to make a decision somewhere along the line and whoever we did that with is going to be a very good player who didn’t deserve it.
"They haven’t [given us problems] because of how professionally they took those discussions and how much respect they showed for the whole group on Sunday.
"As have lots of substitutes who are good players who weren't able to get on the pitch. We know that to be a successful team this is one of the areas that’s got to be right.
“We don’t win Sunday's game without the way players prepared for the friendlies, without the way the players who didn’t make the final squad of 26 contributed through that preparation period.
"All of those factors come into a team getting the sort of result it did. That’s a team – that’s what it’s about. We’ve got to continue that right the way through this tournament."
Southgate acknowledged that England would have to handle mounting pressure after they avenged the World Cup semi-final defeat to Croatia, but played down expectations by pointing to the age of his young squad.
"We had a good day on Sunday and a good performance, but we know what football is and we know how sport works," he said.
"We haven’t achieved our first objective yet. So it’s really pleasing that everybody outside is happy and we can go home, back to St George’s, pleased with the day’s work. But we have to move to the next game.
"Of course, expectations are always going to be there. We have to handle that.
“But as long as that’s realistic. You saw the age of the team, the number of caps compared to the opponent on Sunday.
“ We’re still growing and learning and to be able to bring boys like Jude [Bellingham, 17] into that environment is also hugely encouraging.
“Sunday was a good day. We know we had to start again on Monday morning."