England face Slovakia in the Round of 16 on Sunday in Gelsenkirchen and while the 29-year-old is not expected to start, he could provide a boost off the bench to a squad that has been without a left-footer in his position.
"It's a calculated risk," Southgate told reporters when his squad was announced earlier this month. "I think you can take one gamble and that's a gamble, we've got enough evidence to believe it can pay off."
Defence has been the most positive part of England's performance in Germany, with them conceding one goal in the group stage.
While boasting some of the world's best attackers, England mustered only two goals in three games and were 20th in goal attempts.
Newcastle's Kieran Trippier is right-footed and so tends to turn inside, while Shaw is an attacking defender who loves to make overlapping runs down the left flank much like the team's right back Kyle Walker.
England's two goals by Jude Bellingham and Harry Kane were assisted by Bukayo Saka and Walker on the right.
Shaw was solid in England's 2020 European Championship run, scoring an early goal in the team's loss to Italy in a penalty shootout in the final. At the 2022 World Cup, he was one of six England players to spend the majority of games in the opposition's end.
There is also speculation around Trippier's fitness for Sunday after he sat out a training session earlier this week. A calf injury saw the Newcastle defender miss eight Premier League games near the end of the season. Southgate is expected to start Ezri Konza in his place.
United manager Erik ten Hag called Shaw "the best left back in the world" last month, while acknowledging injuries had been the common theme of the player's career since he suffered a gruesome double leg break in 2015, a career-threatening injury that kept him out of training for a year.
He was also sidelined for several weeks in 2022 when he underwent surgery to remove bolts from the leg.
Hampered by a hamstring injury this past Premier League season, he played 15 games for United, the second lowest in his 10-year career.
"This guy is so good. But he has suffered a horrible injury at the start of his career and because of this he will never be able to play 60 games per season," Ten Hag told Dutch media.
Shaw's last game was on Feb. 18 against Luton Town, which he said he never should have played. It set him up in a race against time to feature for England.
"If the manager asks me to play, I'm never going to say no. But I shouldn't have played (at Luton)," Shaw said recently. "It's kind of everyone's fault. Partly my fault, partly the (United) medical staff.
"I came back too quickly and I actually ended up getting another injury in my hamstring."
Saka, who plays on the right wing for England, scoffed at pundits' suggestions that he should slot in at left back where he played earlier in his career.
"I don't think putting me out of position is the solution," Saka said.
If England beat Slovakia, they will play either Italy or Switzerland in the quarter-finals.