Super Eagles captain John Mikel Obi has given reasons why the team had a poor first half against the Three Lions at the Wembley Stadium.
Man-of-the-match Cahill was playing in an England shirt for the first time since October 2017 and Southgate said: "Gary is deservedly back in the squad with the way he finished the season with Chelsea".
England have one more warm-up game against Costa Rica next Thursday.
14 May was set as deadline for participating member associations to release a list of 30 players, which Nigeria also complied with.
As it turns out, the boos are thought to be the Nigerian fans' way of venting their frustration towards the Spurs midfielder for choosing to play for England instead of Nigeria.
"What we saw in the second half was very positive", the German said.
"The most important thing I do over the next six weeks is protect the players and [make sure] they respect each other and they understand how important it is that we support each other and work together".More news: Three Actresses accused Weinstein of rape
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"The first lesson is we are not ready yet".
England found the going tougher in the second half against an improved Nigeria but held on for victory in their penultimate friendly before the World Cup.
"So, we have to continue to play like this".
"I don't think we got a lot of answers today though because Nigeria just lied down in the first period and let England do what they want".
Barring a late change of heart, that endorsement should see the 24-year-old start his first competitive England match in the Group G opener in Volgograd.
England Manager, Gareth Southgate's having named his provisional 23-man squad for this summer's finals in Russian Federation, will be under pressure to impress in England's final home game at the national stadium before the tournament.
Mikel knows Nigeria have to sharpen up in Russian Federation if they to get through a tough group featuring Argentina, Iceland and Croatia.