The Leicester player was fined NZ$10,000 (£4,800) for wearing the unauthorised mouthguard in England's victories over Argentina and Georgia.
World Cup players are subject to strict sponsorship rules, with players prohibited from displaying brands on scrumcaps, mouthguards and underwear.
Tuilagi's brother Alesana, who plays for Samoa, received a fine for the same offence during a match against Fiji, leading some to believe that Manu's move was an act of support.
"It was a genuine error and he's trying now to focus on Saturday's game," a Rugby Football Union spokesman commented.
England face France in a quarter-final clash on Saturday, October 8.