The Liverpool attacker was somewhat controversially named on the standby list for this summer's tournament despite the fact that he has only made 14 appearances for his club this season due to a number of injury problems.
The 30-year-old could be on the bench for the Reds in the Champions League final next weekend, and Southgate has insisted that the attacker is in his thinking ahead of the tournament in Russia.
"Maybe if we give him 10 days and who knows what happens," Southgate told reporters.
"He might have a big impact in the Champions League final and we have an issue (an injury). The reality of the conversations have had with him is that to find performances on the back of the limited minutes will be really difficult."
Lallana has scored three times in 34 appearances for the England national team.