Wilmots's side won their friendly before the World Cup 1-0, with Dries Mertens struck the winner in the 89th minute, but the Chelsea striker appeared to be injured in a tackle from Tunisia's Bilel Mohsni.
Lukaku was brought off in second-half stoppage time but Wilmots claimed youngster had suffered, at worst, a "severe sprain" and should only miss three or four days of training.
"Romelu has problems walking," he said after the game. "His injury seems at first sight not too bad, the doctors told me.
"This is preliminary to a severe sprain but we will obviously do additional tests on Sunday. For the time being, Romelu will miss three to four days, hopefully it stops there."
Lukaku is expected to be his country's main striker in Brazil with the 21-year-old having scored four goals in his country's three lead-up games, on the back of 16 in all competitions on loan at Everton last season.
Despite scoring, Mertens was less positive about Belgium's performance but paid tribute to Nacer Chadli and Vincent Kompany for their passes, which created his goal.
"It was not a good match," Mertens said. "We won 1-0 and that is ultimately what it's all about but we had to play faster.
"Scoring is always good. The pass from Chadli was tailor-made, like the first pass from Kompany."