Son was targeted by chants of “DVD” and “he sells three for a fiver” in relation to South Korean street salesmen, with reporters at the match reporting the abuse shortly before half-time.
Son managed to silence his abusers in the best possible way by scoring a hat-trick in the 6-0 demolition, but the Football Association could look to take action against the League One club if the abuse is reported.
Millwall are already facing an investigation for the crowd trouble that followed the FA Cup fifth round victory over Leicester City last month, where a pitch invasion led to a section of Millwall fans trying to reach the away supporters at The Den, forcing police to keep them separated and horses to be used on the pitch in an effort to force them off the turf.
The Millwall manager, Neil Harris, condemned the chants after the match, and called for supporters of the club to cut out such disgraceful abuse of opposition players given that it wouldn’t be accepted outside of football either.
"We don’t condone it,” Harris said. "It's wrong in society, and it's wrong in football.”
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