The Three Lions boss named a provisional 33-man group last week for this summer’s tournament.
It is less than a fortnight until the Three Lions face World Cup semi-final foes Croatia at Wembley, but a complicated build-up means the ex-defender has yet to finalise his group.
Southgate will not have been able to work with a third of his 33-man provisional squad due to continental cup final commitments by the time he finalises his Euro 2020 choices on Tuesday.
The fitness of Harry Maguire and Jordan Henderson are key considerations along with the balance of his group, especially given the strength in depth at right-back.
Southgate last week made the late decision to name an expanded squad rather than his 26-strong selection, which he will now do ahead of Tuesday’s UEFA deadline.
It means seven players will be axed from the group on the eve of Wednesday’s warm-up friendly against Austria in Middlesbrough where those already in camp will remain to aid preparations.
The newly called-up trio of Aaron Ramsdale, Ben Godfrey and Ben White look most likely to miss the Euros, while the position of Ollie Watkins, who made his debut in March, also looks precarious.
Mason Greenwood is involved for the first time since being sent home with Phil Foden following his debut in September, while versatile Bukayo Saka only has four caps to his name.
Southgate’s central midfield selection could hinge on the fitness of Liverpool captain Henderson.
The 30-year-old joined up with the squad on Saturday having been out since undergoing groin surgery in February, with Kalvin Phillips also with the group after sustaining a shoulder complaint.
World Cup 2018 veteran Jesse Lingard was another to report to Rockliffe Hall in the north-east but faces tough competition, having been fast-tracked back into the fold in March due to injuries to other attacking options.
The severity of Maguire’s ankle injury remains shrouded in mystery, the Manchester United captain having failed to train on the eve of the Europa League final and then being an unused substitute in the loss to Villarreal.
Another of the key issues comes at right-back, with Southgate having four elite options in his 33-man party.
Reece James shone in Chelsea’s Champions League final win against Manchester City, whose right-back Kyle Walker is one of just three players in their thirties in the expanded group.
Another of those is Atletico Madrid’s LaLiga-winning full-back Kieran Trippier, while Liverpool star Trent Alexander-Arnold was the odd man out in March when omitted from England’s World Cup qualification triple-header.
“I could be drawn into talking about any individual and then get myself into a mess,” Southgate said when asked about Alexander-Arnold’s recall last week.
“I know people think I have an obsession with right-backs. I just see four good footballers. We go into a tournament with all sorts of flexible players that can fill different roles.”