England boss could be without Arsenal forward for 2018 World Cup qualifier, but has welcomed back Jack Wilshere and Harry Kane to the Three Lions fold
The Arsenal attacker's wife is expecting to give birth to the couple's second child, meaning he is primed to leave the squad at short notice if required.
England face rivals Scotland in World Cup qualifying at Wembley on Friday, before hosting Spain in a friendly four days later.
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"We have named 25 because there are a couple of players with knocks from yesterday," Southgate said at a media conference on Monday.
"We are not totally sure but Theo Walcott could disappear at any time with his wife expecting and we are conscious of that.
"We are happy with where everyone is at, two exciting games and will need to use the squad for both of those matches."
Dele Alli is out with a knee injury, but his club team-mate Harry Kane is back after making a goalscoring return from an ankle problem in Tottenham's draw with Arsenal on Sunday.
"The first thing I have to do with Harry is check how he is after yesterday," said Southgate.
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"Spurs, with Alli missing, took a big decision [to play Kane] and we need to see the level he is at. It is great to see him back scoring and with a smile on his face.
"Alli is a super player - I like him a lot and have worked with him since he has been in the Under-18s.
"It is a shame for us for him to miss [the Arsenal game] and two matches with England, but we are fortunate to have a lot of good players in that area of the field."
Jack Wilshere, on loan at Bournemouth from Arsenal, has been recalled to the squad and is in line to play for the first time since Euro 2016.
Southgate said: "There has never been any questioning his quality, I feel he is a player that can play at the highest level.
"He has had three 90 minutes for Bournemouth and it is a good moment to invite him back into the squad."