The England manager said that although Cahill remained in contention for a World Cup spot if he can regain his place at Chelsea, he felt it was prudent to assess Tarkowski and Mawson.
"I was really pleased with how we defended during the games in November", he explained.
"Tarkowski and Mawson are players you want to look at for now and beyond the summer".
The government says British ministers or members of the British royal family will now attend the World Cup in Russian Federation in June and July.
Pope is one four 'keepers included alongside Jack Butland and Jordan Pickford.
"Clearly the number one jersey is up for grabs", he added.
There is a return, however, for Arsenal's Jack Wilshere and Manchester United's Ashley Young.
While the notion of England withdrawing from the tournament was played down following foreign secretary Boris Johnson's initial remarks on the matter, the Football Association has confirmed it will continue to work closely with the government with regards to participation. "He's getting closer to where we want him", he said. His technical ability in the middle of the park will be very useful. Midfielders: Eric Dier, Dele Alli, Jesse Lingard, Lewis Cook, Adam Lallana, Jordan Henderson, Alex Oxlade-Chamberlain, Jack Wilshere, Jake Livermore. "He's comfortable picking the ball up anywhere and we want to play through the midfield".
FORWARDS: Sterling, Welbeck, Vardy, Rashford.
"As far as all my briefings and information is, we're preparing to go to the World Cup, there's no doubt in our mind that's what we should be doing", Southgate said on Thursday. "Danny comes in as a replacement for Harry as a player who can run in behind".
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