Manchester United defender Luke Shaw has been included in the provisional squad, despite not featuring since 18 February because of injury.
Shaw is the only recognised left-back to be called up but Newcastle’s Kieran Trippier and Liverpool’s Joe Gomez have both filled in previously for the Three Lions.
“[Shaw] is up against it – he has missed a lot of football,” Southgate said.
“Because he has been our first-choice left-back we are giving him more time. I am not sure what is realistic at this moment in time.
“I have talked with him and the medical staff and I have liaised with Manchester United very closely. I have to say he is a long shot.”
Chelsea right-back Reece James has also had a difficult time with injuries this season, only returning from a lengthy spell on the sidelines on 11 May, and he has been overlooked.
“This has been as complicated a group as I can remember picking,” Southgate said.
“There are a significant number injured – that will be apparent looking at the players who have missed the last few games of the season and some who are coming back for the last few games of the season.
“Some players are going to need more work, other players don’t need much at all and need refreshing. We have never had so many unknowns. The key is making the best possible decisions.”
Southgate has selected 11 defenders in his squad and 12 forwards, though that includes all England’s wide attacking options. He confirmed that number would have to be reduced by the deadline at midnight on 7 June.
“I think you can take too many forwards,” he added. “You can have too many options in certain areas of the pitch. Of course, with forwards you’ve got to give them love and make them feel special.
“If you’ve got too many, that can also be complicated. There’s a lot to think about over the next couple of weeks. I’d have loved it to be cleaner and more definitive, but it’s not the situation we’re in.”
Talisman Harry Kane missed Bayern Munich’s final two games of the season with a back injury but is expected to be fully fit by the time England kick off their Euros campaign against Serbia on 16 June.
Kane has enjoyed a stellar first season with the German club, scoring 44 goals, and is England’s all-time leading goalscorer.
“As far as we can tell that seems to be clear now,” Southgate said when asked about Kane’s fitness.
“I don’t know what the possibilities of recurrence are, but the advice is that should be all calm.”