Having qualified for the FIFA World Cup from UEFA Group A without losing a game, Belgium were defeated for the first time in 10 matches by Colombia on Thursday.
Wilmots' only experimentation against Japan in Brussels will come in the form of individual players, rather than wider team selection.
"This time is no longer about testing," he told a press conference on Monday.
"My team is clear in my head but I think I'll try something different with one of my players.
"I'll talk to him, but I think he could be even stronger in this role."
Christian Benteke and Nicolas Lombaerts remain doubts for the friendly in the Belgian capital, while Nacer Chadli has been ruled out.
"Christian Benteke is still suffering from a bruised knee," Wilmots continued.
"They put tape on it, but it still hurts when he strikes the ball. We'll see tomorrow (Tuesday) and decide for him as well as for Nicolas Lombaerts, who has an ankle problem.
"(Chadli) trained with us. He is on the road to recovery but he will not play (against Japan)."
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