Roberto Martinez revealed that Romelu Lukaku is an injury doubt for Belgium's World Cup blockbuster tie against England after the in-form striker picked up an ankle injury on Saturday.
Lukaku and Eden Hazard both netted twice as the Red Devils routed Tunisia 5-2 in Moscow to leave them top of Group G and on the verge of the last 16.
But the Manchester United forward came off in the second half at the Spartak Stadium in Moscow.
"We need to look at Lukaku's ankle. He took a knock and has to be assessed," said Martinez, who revealed man-of-the-match Eden Hazard also picked up an injury.
"Eden Hazard has a calf knock, Dries Mertens has an ankle problem. There will be major changes against England.
"If we could have seven days to prepare, I'd say we'd start with the same team, but the reality is that you are only as good as the 23 players."
Lukaku's absence would be a blow to Belgium's hopes against England, with top spot in the group likely to be up for grabs.
The United player now has four goals at Russia 2018, making him joint top scorer alongside Portugal's Cristiano Ronaldo.
"He was incisive, he was confident and in front of goal he was as clinical as you would expect," said Martinez. "But like any other player he knows well that what matters is improving as a team."
There was some good news for Martinez, with Chelsea midfielder Hazard expected to be fit to face England in Kaliningrad on Thursday.
"I felt something in my calf in the first half but nothing big and I think I will be OK tomorrow," said Hazard.
Ominously for England, Martinez said there is more to come from a Belgium team growing in confidence, but who are far from the finished article.
"We are still growing and in certain situations we didn't control the ball well enough," said Belgium's Spanish coach.
"We are a team who link up well when we are in front of goal and there were good combinations, but we still have work to do."
Martinez says while the nation should celebrate the victory, the team's focus is now on facing England.
"It must have been a great game to watch as a neutral but there are lots of things we need to work on and that starts tomorrow," he said.
Belgium forward Romelu Lukaku scored twice against Tunisia in a 5-2 win