England defender Harry Maguire was called up to the Nations League squad on Tuesday despite the Manchester United captain having last week been detained by police over a brawl on the Greek island of Mykonos.
Centre back Maguire pleaded not guilty and was released from police custody after his arrest on Thursday. The 27-year-old was being represented by his lawyers in court at a hearing on Tuesday.
"It is clearly a decision that is not straightforward and a decision that I could only take on the information I have," Southgate told a news conference after selecting Maguire.
"I have spoken to Harry, I have the insight to the story which is very different to what is being reported. Clearly if facts change or information changes, I have to review that decision.
"Harry's been an outstanding character for us. I've no reason to doubt what he's telling me and it's important he knows he's got my support at this moment."
But the England manager said Maguire did have regrets about how the incident had affected the team.
"Harry regrets the fact he has brought that sort of focus and attention to the team - he apologised for that - but also has his own side of the story," added Southgate.
England captain Harry Kane is available for the Nations League matches against Iceland and Denmark next month despite currently being in quarantine following a trip to the Bahamas.
Manchester United forward Mason Greenwood, Manchester City's Phil Foden and fellow midfielder Kalvin Phillips of Leeds United will all have chance to make their international debuts.
Goalkeeper Dean Henderson, back at Manchester United after a loan spell with Sheffield United last season, also earns a call-up along with regular first-choice keeper Jordan Pickford and back-up Nick Pope.
Midfielder Jack Grealish, who helped Aston Villa retain their Premier League status last season, has not been included in the squad nor has Burnley defender James Tarkowski.
With Ben Chilwell and Luke Shaw injured, there is no recognised left back in the squad, meaning one of the right backs will have to switch sides, which Southgate conceded was "not ideal".
His England side have not played since November last year due to the disruption caused by the COVID-19 pandemic, which also forced Euro 2020 to be pushed back a year.
England face Iceland on Sept. 5 in Reykjavik before taking on Denmark in Copenhagen on Sept. 8.
Goalkeepers: Dean Henderson, Jordan Pickford, Nick Pope
Defenders: Trent Alexander-Arnold, Eric Dier, Joe Gomez, Michael Keane, Harry Maguire, Tyrone Mings, Kieran Trippier, Kyle Walker
Midfielders: Phil Foden, Mason Mount, Kalvin Phillips, Declan Rice, James Ward-Prowse, Harry Winks
Forwards: Tammy Abraham, Mason Greenwood, Danny Ings, Harry Kane, Marcus Rashford, Jadon Sancho, Raheem Sterling