The 32-year-old Kompany, a key part of Belgium’s defence, abruptly left the pitch in the 55th minute of the match after appearing to hurt himself when lunging for the ball.
“You could feel that when he came off we were all very much worried,” coach Roberto Martinez told a news conference, adding that it would probably take 48 hours to determine how serious the injury was.
“He said he felt discomfort in his groin when he walked off the pitch. He’s not a young player, he knows his body. At that point it was the right time to come off,” Martinez added.
The injury was the key talking point after Belgium’s 0-0 draw with Portugal as both sides showed attacking intent but no killer blow less than two weeks before the World Cup begins.
Both Belgium and the European champions had promising phases, with Portugal’s Bernardo Silva in particular creating chances at the end of the first half.
Belgium’s best chance fell to defender Jan Vertonghen, earning his 100th cap, as he forced goalkeeper Beto into an acrobatic save midway through the second period.
Belgium began with their strongest possible 11, apart from the injured Axel Witsel and Thomas Vermaelen, and a trial three-man defence.
Portugal coach Fernando Santos made six changes from the team that started Monday’s 2-2 draw with Tunisia and were again without Cristiano Ronaldo, still resting after last weekend’s Champions League final. Striker Andre Silva started on the bench, giving 21-year-old Goncalo Guedes the chance to shine in attack.
Santos expressed satisfaction after the match, saying his team had done well to keep a clean sheet against a team that set a joint record of goals in qualifying for the World Cup.
The hosts made five second-half changes to try out some of their fringe players before Martinez cuts his 28-man squad to 23 on Monday.
“I cannot be fully satisfied because of Vincent Kompany’s injury but apart from that it was the test we needed,” he said, adding that Vermaelen at least appeared to be returning to fitness.
Portugal host Algeria on June 7 before their World Cup opener on June 15 against Spain in a group also featuring Morocco and Iran.
Belgium will play further friendlies against Egypt on June 6 and Costa Rica on June 11. They open their World Cup campaign on June 18 against Panama, before also facing England and Tunisia in Group G.