The 21-year-old impressed in Wednesday's memorable Champions League win against Real Madrid, grabbing a brace as Spurs triumphed 3-1 at Wembley.
However, Alli missed Sunday's Premier League encounter against Crystal Palace with a hamstring injury that the Football Association has confirmed will keep him out of this month's internationals.
England host Germany in their first World Cup preparation friendly on Friday nighgt, with Brazil arriving at Wembley the following Tuesday.
A tweet from @England read: "Squad update: the #ThreeLions will be without @dele_official for this month's games. Get well soon, Dele!"
Alli's absence means Southgate now has 24 players for the Wembley double-header, although there could yet be more withdrawals after Sunday's matches or when the squad convenes at St George's Park on Monday.
All is odds-on at 1/12 with Sky Bet to be part of England's World Cup squad.
The England manager has plumped for a bold squad for the upcoming matches.
Tammy Abraham (100/30 for the World Cup squad), Ruben Loftus-Cheek (4/1) and Joe Gomez (9/2) have been called up by England for the first time, but Daniel Sturridge (5/2), Alex Oxlade-Chamberlain (4/5) and Chris Smalling (11/4) are among those to have missed out.
Tottenham boss Mauricio Pochettino later revealed Harry Winks is a doubt for England's friendlies after twisting his ankle in Tottenham's 1-0 win over Palace.
Pochettino said: "He twisted his ankle. It's a little bit painful.
"We must assess him with our medical staff and with the national team medical staff too. He's so excited to play for the national team. I don't know (if he can play). It's about him and both (teams of) medical staff.
"Dele Alli against Manchester United finished the game feeling his posterior tendon, close to the hamstring, and then played 90 minutes against Real Madrid.
"It's a very, very, very small thing. It was a massive risk (to play him) again. We're sure he'll be ready for Arsenal (after the international break). We hope he's going well in the next few weeks and will be available."
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