Jude Bellingham, who scored England’s winner against Serbia in their opening game, turned 21 on Saturday and celebrated with the squad.
Southgate dismissed concerns over the Real Madrid player’s fitness and says he will be better for playing in the group games.
Southgate said: “Jude missed a period at the end of the season with an ankle injury so he will have benefited from the matches he has had.
“Of course we have to be mindful of the quality we have to come into the game; freshness might make a difference in the latter stages of matches. But I am not concerned about Jude Bellingham’s condition. His birthday was a big day for him and a reminder of how well he deals with the expectation around him at a remarkably young age.
“There are few players who have had to live in that world. His world is different to most 21-year-olds.”
England captain Harry Kane, who scored four goals in the knockout stage of Euro 2020, backed Bellingham and the rest of his team-mates to shine.
Kane told BBC Radio 5 Live: “There is no need to panic.
“Everyone is coming into their full sharpness into the knockout stages now, which is the most important thing.
“We are both ready. I know Jude is ready to get back out there tomorrow and show the world what he has been showing all season.”