Alli's availability would be a boost and his participation at England's base in Repino suggests the midfielder is close to recovering from a thigh strain he picked up in the side's opening win against Tunisia.
But with qualification already secured and a strong argument emerging for finishing second rather than first in Group G, Southgate must decide whether key players, including Alli, are in need of a rest or if he sticks to a winning formula.
While the eleven that started Sunday's match against Panama were spared training after their crushing 6-1 win, England's substitutes were put through a running session and did some ball work.
Ruben Loftus-Cheek, Alli's replacement against Panama, was bright and composed in Nizhny Novgorod, his shot cannoning off Harry Kane and into the net for England's sixth.
But Alli possesses a goal threat that Loftus-Cheek lacks and, crucially, England need if they are to take the burden away from their free-scoring captain.
"It's about the balance of goalscorers in the team," Southgate's assistant Steve Holland said last week.
"Someone told me a long time ago that to have a successful team you need three-and-a-half goalscorers. We've tried to address that."
That would suggest Alli will remain first choice and Southgate may be keen he plays against Belgium to ensure he gets minutes on the pitch before the last-16 match.
Also in Southgate's thinking may be the threat of suspensions. Loftus-Cheek picked up a yellow card against Panama and another would bring a one-match ban, with Kyle Walker is also vulnerable after being cautioned against Tunisia.
The less drastic approach might include a start for Danny Rose, short of match fitness after an injury-affected season at Tottenham, in place of either Kieran Trippier, who has a knock, or Ashley Young.
Manchester United forward Marcus Rashford has been pressing his case and could also come in for Raheem Sterling.
England cannot afford an injury to Kane. Fleet-footed Leicester striker Jamie Vardy is waiting in the wings but Kane's hat-trick against Panama put him in pole position in the race to be topscorer in Russia and he will want to add to his tally.
Southgate also has the option to rotate more heavily. Eric Dier could replace Jordan Henderson as the midfield anchor, Gary Cahill and Phil Jones could come into the back three, and even Jack Butland could be given a run-out in goal.
But so much upheaval seems unlikely, even if Southgate has said before that he would not wish on anyone his own World Cup experience of 2002, when he was left unused by Sven-Goran Eriksson.
Defeat to Belgium would not be disastrous -- it could even be desirable in terms of the draw -- but England might find it hard to escape the conclusion they had lost to their first elite opponent after overcoming two poor ones.
"The harmony of the group is important to me," Southgate said on Sunday. "Some have been a couple of weeks without a match and we're going to need more people ready as we get further into the tournament.
"But we want to keep playing well and keep the consistency in our play, too. We want to keep the momentum going and keep progressing as a team."