Bayern Munich beat Manchester United 3-1 back in 2014
However, he missed the 3-1 Bayern win after a ticketless United fan crashed into him after jumping over the ticket barrier.
Rettensteiner fractured both his kneecap and elbow as well as losing a tooth in the fall.
He was then treated in hospital, where his injuries were so severe he still uses a walking stick nearly three years on.
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A Bayern Munich fan was injured prior to their clash against Manchester United
The United supporter in question was caught but it is unknown what action was taken against him.
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Rettensteiner is suing Bayern for £103,000, claiming the Bundesliga side failed to implement safety measures to prevent the incident from happening.
On Wednesday, his lawyers argued to a Munich court that routine checks or taller turnstiles would have stopped the fan from gaining access to the ground.
Bayern have hit back by claiming the incident was a one-off occurrence.
Manchester United fell apart at the Allianz Arena in David Moyes' final European game in charge
“The Allianz Arena is one of the safest stadiums in the world,” Bayern lawyer Gerhard Riedl argued, according to Bild.
“There is no security risk, this was a regrettable accident.”
The United clash was viewed as a "high risk" game, according to Bayern, and they could not confirm whether security checks had been conducted on the day of the incident.
Bayern beat United 3-1 that day courtesy of goals from Mario Mandzukic, Thomas Muller and Arjen Robben.