Footage shared widely on social media appeared to show the 22-year-old Ivory Coast international raising his middle finger while outside a hotel with his teammates in Kuala Lumpur earlier this week.
"I have respect for people but not for the one who insults my mom," Amad wrote on X. "I shouldn't have reacted like that but I don't regret what I did. We had a great time in Malaysia with good people."
United did not respond to a request for comment.
The team's post-season Asia tour had already got off to a bad start when United lost their friendly against a South East Asian All-Star team 1-0 in Kuala Lumpur on Wednesday. Some fans booed in response.
Even Malaysian prime minister Anwar Ibrahim was disappointed, suggesting United could be relegated after watching the team lose, despite the fact their performance was probably affected by jet lag.
"Ouch. Man United failed to win another trophy," Ibrahim, one of 72,500 fans at Bukit Jalil Stadium in the Malaysian capital, wrote on social media in a post translated from Malay.
"Looks like you're in for another tough season, hope you don't drop to the Championship," he added, referring to England's second tier.
United have travelled on to Hong Kong for another friendly on Friday.
"We were not really focused in the last game, and everybody can feel it," United manager Ruben Amorim said on Thursday. "We just want to turn the page and improve in the next season."
United finished 15th in the Premier League, their lowest-ever ranking in the competition, when the season concluded on Sunday, just four days after a loss in the Europa League final to Tottenham Hotspur.
