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James was the standout performer on his full Three Lions debut against Denmark at Wembley in October last year, but tarnished the occasion when a few choice words to the referee after the full-time whistle had him seeing red.
Southgate was quick to praise James - who has not missed a squad since - after the game, but the 21-year-old had his concerns.
"Of course [I thought it could cost me], I was very upset at the time," said James.
"It was my first start and getting sent-off is not the best look.
“As you said, it was a mistake, in life mistakes happen and I had to learn from it."
There were quick lessons to learn for James.
The defender has experience of stricter officiating styles in the Champions League with Chelsea, but feels international football is on another level - though the incident is something he is pleased to have got out of his system since.
"I learnt that European referees are a lot stricter. I felt it was very harsh to get sent off, but I learned my lesson and I know I can't afford to do anything like that again in this tournament because it would be very costly.
"I think in the Premier League the rules and the referees are a lot more lenient. When you come into international football the referees are a lot stricter. I didn't really know that before I got sent off, so maybe it was a good lesson to learn.
“If I didn't learn it then it could maybe have happened at a major tournament like this."
The red card was not the only hurdle for James over what has been a successful, but challenging, season.
As a Chelsea academy graduate, James benefitted from Frank Lampard's tenure and looked to have overtaken Cesar Azpilicueta in the pecking order at Stamford Bridge for good. But when Lampard was sacked and Thomas Tuchel arrived, the youngster found himself with a fight on his hands.
"We had Frank Lampard and I was very lucky to play the majority of the games under him, but when our new manager came in our style of play and formation changed," said James.
"The first few games I found myself out the team. That was quite difficult to deal with. I had to adapt and learn. It’s hard when a new manager comes in mid-season, I’ve never had it before. It was a big learning curve.
"You have to accept the decision and work hard and try and prove to the manager that you should be playing and you're definitely capable of playing.
"These moments come around quite often in football. Things happen so fast, I don't really get advice from anyone. I was just wanting to train and carried on working hard, and when I got more opportunity I knew I had to take it."
Those two moments in his career - a red card for England and being knocked down at club level - have helped shape James into a full-back Southgate has complete trust in ahead of the summer.
Deliver on that trust and help England to a memorable summer, and those two incidents will end up as mere footnotes in what will go down as an astounding season for James.