The 22-year-old Ivorian winger was seen raising his finger when the team were walking past fans in Kuala Lumpur, where they lost to the ASEAN All-Stars on Wednesday.
"I have respect for people but not for the one who insults my mom. I shouldn't have reacted like that but I don't regret what I did," Diallo wrote on X on Thursday.
"We had a great time in Malaysia with good people."
Diallo had come on as a substitute for the second half of the loss. The players were also booed after the defeat and United boss Ruben Amorim said his team, who finished 15th in the Premier League, cannot hide from their fans.
United wrap up their post-season tour with a match against Hong Kong on Friday.