England midfielder Dele Alli suffered a slight thigh strain in their opening 2-1 Group G win over Tunisia and the injury will have to be managed over the coming days, the FA said on Twitter.
The 22-year-old seemed to pull up with a muscle problem in the first half, with Fabian Delph getting ready to come on, but Alli managed to continue until being taken off after 80 minutes.
The Tottenham Hotspur player had a scan on Tuesday while his teammates trained after picking up the strain in Volgograd.
"We had to keep monitoring Dele," England manager Gareth Southgate said on Monday.
"He was feeling a little bit of an issue just before halftime, but he felt he could carry on."
England play their second match on Sunday in Nizhny Novgorod against Panama, who lost 3-0 to Belgium in their opener.
Tunisia goalkeeper Hassen out of World Cup
Tunisia goalkeeper Mouez Hassen's World Cup is over after just 16 minutes of action due to an injured shoulder suffered in Monday's 2-1 loss to England, team officials said.
Hassen was forced off early in Volgograd and will not recover in time to continue in the tournament. He will instead return to his French second division club Chateauroux for further treatment.
The injury occurred when Hassen collided with England midfielder Jesse Lingard and he was replaced by Farouk Ben Mustapha, who conceded a last-minute Harry Kane goal as England won the Group G encounter.
French-born Hassen switched his international allegiance to Tunisia earlier this year and made his international debut in March. He had previously played for France at under-21 level.