The Three Lions boss is getting his first chance to see his 23-man squad in match action against another qualified side ahead of their tournament opener against Tunisia on June 18.
Southgate has confirmed that Jesse Lingard will start against Nigeria, but insists that the Manchester United man is not directly vying against Tottenham Hotspur's Dele Alli for a starting berth in Russia.
"I don't think these are trial matches in that a bad performance rules someone out, but there are opportunities for people to really stake a claim in areas of the pitch where there is huge competition for places," Southgate told Sky Sports News.
"[The goalkeeping berth] will become apparent over the next five days. They are inexperienced in terms of senior international games but Pickford and Butland have the perfect path in terms of junior international tournaments and a lot of Premier League football under their belts."
Following the game against Nigeria, England wrap up their pre-World Cup friendly programme with a clash against Costa Rica on June 7.