Everton defender Michael Keane has been drafted into England's squad to face Germany and Brazil as cover for Gary Cahill.
Cahill will be assessed by Chelsea's medical team before joining up with the national team after picking up a knock during the Blues' 1-0 win over Manchester United on Sunday.
England 20/1 to win 2018 World Cup
Central defender Cahill is not the only concern for Southgate, with Tottenham duo Harry Kane and Harry Winks also delaying their arrival.
Both Kane and Winks were substituted during the win over Crystal Palace and Mauricio Pochettino has called on England to look after his stars so they do not miss any club matches after the international break.
The Three Lions are already without Dele Alli due to a hamstring injury, the midfielder a surprise omission for Spurs against Palace.
Southgate must also wait on the fitness of Liverpool skipper Jordan Henderson due to a thigh problem.