Hummels came off at half-time in Germany's sensational 7-1 semifinal victory against Brazil on Tuesday with Die Mannschaft already 5-0 ahead.
The centre-back said the problem was not major -- but it may keep him out of Sunday's final, when Germany face the winners of the match between Argentina and Netherlands.
Hummels hurt his left knee in Germany's opening match against Portugal but returned for the next game.
He also sat out their round-of-16 match against Algeria with flu, but returned to score the only goal in their quarterfinal win over France.
"More problems with my knee," Hummels told German radio ARD. "We will find out the exact diagnosis [on Wednesday].
"There won't be a horror diagnosis, and there is hope that it will be working again in five days. I will definitely need intensive treatment.
"If I can't train on Saturday, I wouldn't go and say I can play on Sunday."
Hummels helped set up Germany's fifth goal on Tuesday with a dashing run through midfield.
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