A breach of coronavirus isolation rules while in Iceland last month has proven costly for Manchester City playmaker Foden and Manchester United forward Greenwood, but the England door will be open to them in future.
That means both could still play a key role for the Three Lions at next year's delayed Euro 2020 finals.
Foden, 20, and Greenwood, who turned 19 on Friday, appeared to be shown in a Snapchat video posted by one of the women they were said to be socialising with at England's team hotel in Iceland, a meeting forbidden under the country's strict rules in response to COVID-19.
Both were sent home and missed a subsequent game against Denmark, with Southgate taking a dim view of the episode.
Speaking as he named a 30-man squad for the October triple-header, Southgate was asked about the conspicuous exclusion of Foden and Greenwood.
He said: "They're two very talented players. I think with that happened there's two parts: one is we have to send a message to all the players in our teams at every age level that that sort of thing isn't how we do things with England, but then we also have to rehabilitate these two boys.
"Young people will make mistakes. I do a lot of work with the Prince's Trust [charity] who are brilliant at helping youngsters to get back on their feet when things haven't gone right.
"These lads have suffered now enough. We didn't think it was right - I think they need this period of reflection - but then that gives them the opportunity to get back on their feet with their clubs, enjoy their football again.
"They're both starting to play well again which is important. Then I can speak to them after this camp and it will just be about their form as to whether they're selected next time round."
Harry Maguire was recalled, the Manchester United captain having been picked by England and then dropped ahead of last month's games after he was originally found guilty of assault following an incident on holiday in Mykonos, Greece.
Maguire's legal representatives lodged an appeal, meaning the court's initial verdict was nullified and a full retrial will now take place at a more senior court, with Southgate saying there was "no reason we couldn't pick him."
England plays Wales in a friendly on 9 October (AEST) before Nations League games against Belgium three days later and Denmark on 15 October, with all the fixtures being played at Wembley.
England squad in full:
Dean Henderson (Manchester United), Jordan Pickford (Everton), Nick Pope (Burnley); Trent Alexander-Arnold (Liverpool), Ben Chilwell (Chelsea), Conor Coady (Wolves), Eric Dier (Tottenham), Joe Gomez (Liverpool), Michael Keane (Everton), Harry Maguire (Manchester United), Ainsley Maitland-Niles (Arsenal), Tyrone Mings (Aston Villa), Bukayo Saka (Arsenal), Kieran Trippier (Atletico Madrid), Kyle Walker (Manchester City); Jack Grealish (Aston Villa), Jordan Henderson (Liverpool), Mason Mount (Chelsea), Kalvin Phillips (Leeds United), Declan Rice (West Ham), James Ward-Prowse (Southampton), Harry Winks (Tottenham); Tammy Abraham (Chelsea), Harvey Barnes (Leicester City), Dominic Calvert-Lewin (Everton), Danny Ings (Southampton), Harry Kane (Tottenham), Marcus Rashford (Manchester United), Jadon Sancho (Borussia Dortmund), Raheem Sterling (Manchester City).