Henrik Larsson's son Jordan was attacked by Helsingborgs fans after the club's relegation from the Swedish top flight.
A goal from Jordan with eight minutes remaining of the play-off second leg against Halmstads looked set to keep the seven-time Allsvenskan champions up.
However, Marcus Mathisen scored a late brace for the visitors to Olympia, securing a 2-1 win and 3-2 aggregate victory.
Ugly scenes marred the post-match, as masked fans surrounded Jordan before removing the 19-year-old's shirt.
Jordan, involved in a verbal altercation with at least one fan, then had to be escorted away from supporters.
Coach Henrik, the Sweden and Celtic great, was unaware of the incident.
"I didn't see that they attacked Jordan but if I had, I would have gone in there towards him," he said, via the BBC.
"I was not going to run from there. If they turn on me, they turn on me. I had been beaten."