In Belgium's final friendly before the FIFA World Cup, Dries Mertens struck the winner in the 89th minute as the hosts triumphed 1-0 in Brussels.
Wilmots labelled the game 'weird', after it was suspended midway through the first half due to a severe hailstorm and did not restart for 45 minutes.
But the focus after the match was on Lukaku's right ankle, which appeared to be injured in a tackle from Tunisia's Bilel Mohsni.
Lukaku was brought off in second-half stoppage time but Wilmots claimed the Chelsea forward had suffered, at worst, a 'severe sprain' and should only miss three or four days of training.
"Romelu has problems walking," Belgium's coach said.
"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 Belgium's main striker in Brazil with the 21-year-old centre forward 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.
With Tunisia playing five men in defence, Belgium struggled to break through the visitors at the Stade Roi Baudouin, although Wilmots was pleased with his team's patience.
The 45-year-old coach had set up the friendly against Tunisia to prepare for Belgium's World Cup opener against Algeria but he claimed the Algerians would attack more in Belo Horizonte on June 17.
"It was hard to play (against a team) that remained with nine men behind the ball," Wilmots said.
"This was a very good test with an eye on the World Cup and Algeria, although that team has even more talent offensively."
Mertens was less positive about Belgium's performance, although he 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."
