Uncapped Bournemouth striker Callum Wilson has been included in the England squad for the friendly against the United States and the Nations League match against Croatia, the Football Association has announced.
Wilson, who has scored six Premier League goals so far this season, is rewarded with a maiden call-up to the senior ranks.
"He's a threat in terms of running in behind defences," said Southgate.
"He's been involved with either scoring or assists, with a high number of goals this season, so it's a good opportunity for us to have a look at him and see how he fits into what we do, and for him to get a feel of working with the senior team."
Midfielders Dele Alli and Jesse Lingard return after being ruled out of last month's Nations League double-header against Spain and Croatia through injury - with youngsters Mason Mount and James Maddison dropping out.
Alex McCarthy, Luke Shaw and Danny Welbeck are also included, having been forced to withdraw from the previous squad due to minor injuries.
Everton defender Michael Keane is included in an England squad for the first time in 12 months, with uncapped Brighton centre-back Lewis Dunk also retaining his place.
Southgate was eager to defend Rooney's inclusion.
"There have been ongoing discussions since Wayne retired about paying tribute to him and his England career," said the manager, who added Rooney had declined an opportunity to be honoured ahead of the World Cup so as not to take the attention away from those going to Russia.
"Those discussions started 12 months ago. We have an opportunity with the game at Wembley to pay tribute.
"I am still able to look to the future with the squad we've picked, but we've talked a lot about the history of the shirt.
"We have brought former players back to speak and present shirts. I think all the players respect Wayne's contribution and that he deserves the best possible send-off."
Referring to the criticism Rooney's selection has received, Southgate said: "I understand it's caused a lot of debate and conjecture, but it's a way of appreciating what he's given."
Asked if the game was at risk of being a testimonial, Southgate cited the March 2017 meeting with Germany, where the hosts honoured the retiring Lukas Podolski.
"We went to Germany and they honoured Podolski, he started the game and we were all struck by the fact that it was a good way to recognise someone who's had a fantastic career for his country," he said.
"We've not been brilliant at that over the years."
Southgate added that Rooney would not wear the number 10 shirt, and that he hoped "come the night, people will recognise what Wayne has done and he gets the appropriate response he deserves".
England Squad: Trent Alexander-Arnold (Liverpool), Dele Alli(Tottenham Hotspur), Ross Barkley (Chelsea), Marcus Bettinelli(Fulham), Jack Butland (Stoke City), Ben Chilwell (Leicester City), Fabian Delph (Manchester City), Eric Dier (Tottenham Hotspur), Lewis Dunk (Brighton & Hove Albion), Joe Gomez (Liverpool), Jordan Henderson (Liverpool), Harry Kane (Tottenham Hotspur), Michael Keane (Everton), Jesse Lingard(Manchester United), Ruben Loftus-Cheek (Chelsea), Alex McCarthy (Southampton), Jordan Pickford(Everton), Marcus Rashford(Manchester United), Wayne Rooney (DC United)*, Jadon Sancho (Borussia Dortmund), Luke Shaw(Manchester United), Raheem Sterling (Manchester City), John Stones(Manchester City), Kieran Trippier (Tottenham Hotspur), Kyle Walker (Manchester City), Danny Welbeck(Arsenal), Callum Wilson (AFC Bournemouth),Harry Winks (Tottenham Hotspur)
*USA Fixture only**