In the celebrations that followed City's opening goal the man fell over the front wall of the stand occupied by the Cardiff fans and dropped about 20 ft onto the lower tier. The game, which ended 3 - 3, contiunued as an ambulance entered that corner of the ground and he was treated by medical staff for aroud 20 minutes before being stabilised and taken to hospital.
The Millwall fans who thought it was funny and taunted the accident should be ashamed of themselves.
Full details are yet to emerge but it's thought he suffered neck and shoulder injuries and possibly a couple of broken ribs.
Both Millwall and Cardiff shared their concerns and said their thoughts were with the young man.
Dave Jones admitted after the game he was concerned only with the fan and not the result.
He said: “I couldn’t give two hoots about the result after that. One or two of the players saw it and Craig Bellamy said it made him feel physically sick.
“We don’t know much about him but all we’ll say is that we as a club send our best wishes for a speedy recovery.
“We desperately want to do well for our supporters and as a club we have a duty to them. At the moment, we are all just concerned about that guy.”
Millwall boss Kenny Jackett also expressed his concern about the fan.
“We wish the lad well who fell from the stand. I didn’t see it myself but Neil Harris did and said it wasn’t good,” he said.
Millwall's tannoy announcer assured Cardiff fans exiting the stadium that the mans injuries were not life-threatening.
Jay Bothroyd last night tweeted his concerns and wanted to meet the fan.
Jay said: "If anybody knows the person that got hurt today please could they give me his details so I could pay him a visit ...
Ryan Oliver hope you make a full recovery , my thoughts are with you ..."
I was near to the incident but didn't see it happen just saw the looks of concern on the faces of those around me and had the gut wrenching feeling when it dawned on me something really serious had occured.
I wish him well and hope he's back from hospital as soon as possible and on the mend.
Gethin Jenkins told BBC Radio Wales: "Our thoughts are with him and his family at this time. Obviously it puts football into perspective."
He said some players had been affected by what happened.
"Two or three of them seem to have seen it. It was horrendous, coming off the balcony."
Mr Jenkins paid tribute to the emergency services at the New Den.
"They reacted very quickly," he said, "both the police and ambulance guys there.
"We have no complaints. Millwall have been great from that perspective. Let's just hope that everything's going to be okay."