Real Madrid midfielder Jude Bellingham sustained a calf injury in training just over two weeks before England began their Nations League campaign.
The 21-year-old was unable to finish a club session on Friday.
BBC Sport has contacted Real to establish how long Bellingham will likely be out.
Reports in Spain suggest he will be unavailable until late September, which would rule him out of Lee Carsley’s opening two fixtures as England’s interim manager.
England face the Republic of Ireland on 7 September and Finland on 10 September in the Nations League.
In a statement, Real said Bellingham had medical tests on Friday on the problem with his right calf and that his “progress will be monitored”.
Carsley, 50, was announced as interim manager earlier this month following Gareth Southgate’s resignation after Euro 2024.
Bellingham scored 23 goals in 42 appearances during his debut season at the Bernabeu last term as Madrid won a La Liga and Champions League double.
The former Dortmund midfielder started in last weekend’s 1-1 La Liga draw at Mallorca, as well as the Super Cup victory against Atalanta earlier this month.
Carsley is set to name his squad for the Nations League fixtures on 29 August.
He will be assisted by former England left-back Ashley Cole during his interim spell in charge.
Ex-England centre-back Joleon Lescott and the Football Association’s head of coaching Tim Dittmer also join Carsley’s backroom team.