Dele Alli will do "everything in my power" to reach full fitness in time for England's game with Panama on Sunday.
The Tottenham playmaker suffered a minor thigh strain during the 2-1 victory over Tunisia and underwent a scan on Tuesday afternoon, which confirmed the injury.
On Wednesday night Alli tweeted: "Frustrating to have picked up a slight injury on Monday."
"Will do everything in my power to get back to full fitness asap! #ThreeLions"
Alli looked set to come off during the first half of Tunisia game, with Fabian Delph stripped and seemingly ready to come on, but the 22-year-old lasted until the 80th minute when he was replaced by Ruben Loftus-Cheek.
England manager Southgate had kept tabs on Alli's fitness throughout their World Cup opener after the first-half setback.
"We had to keep monitoring Dele," Southgate said on Monday.
"He was feeling a little bit of an issue just before half-time, but he felt he could carry on and I thought the runs that he was making and the way he was pressing the ball was still causing a problem."
England have made a winning start to a major tournament for the first time since 2006 and victory over Panama could be enough to book their place in the knockout stages.
Source: Skysports