Southgate suggested Sancho has been spurred on by his move to Manchester United as he explained the decision to start him against Ukraine.
Sancho, 21, saw his involvement during England's first four Euro 2020 fixtures limited to a brief cameo off the bench, but found himself thrust into the spotlight for Saturday night's quarter-final.
And Southgate declared Sancho's "really bright" displays at the training ground saw him play his way into the starting XI to face Ukraine.
Southgate said, "He's [Sancho] been really bright in training in the last ten days or so.
"We've been looking for those moments in training, where they've been showing aggression with the ball, and he's really shown that.
"It looks like Ukraine are going to have five at the back. It's been a huge challenge for all our attacking players because we have got so much depth.
"We can't keep everyone happy but Jadon Sancho's profile for this game - we need one-versus-one specialists - he has got that ability.
"The Stadio Olimpico is a brilliant theatre. I think it's been good for us to come away from Wembley but we will find out over the next 90 minutes if that's going to be a deciding factor for us.
"We know the weather forecast is extremely hot. It's a lovely night for football.
"Ukraine have had a fantastic run and have already gone further than any of their teams in past in the European Championship.
"We have to be patient at the right times. We need to make short runs and use width to break them down."
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