Video footage emerged on Friday, that day after the game, which showed a man lunge at referee Bjorn Kuipers at the end of the quarter-final.
Contentious: Referee Bjorn Kuipers sent Dynamo Kiev's Andriy Shevchenko off against Braga
With the first leg fixture delicately poised at 1-1, Kuipers saw fit to brandish a second yellow card to Shevcheko shortly after the hour mark, for an offence not dissimilar to that which saw Robin van Persie dismissed against Barcelona last month.
The home crowd was infuriated by the decision after Shevchenko played on and 'scored' after being flagged offside. He said later he had not heard the referee's whistle.
The attack at the end of the match appeared to be felt by Kuipers, who wheeled round in disbelief. UEFA are yet to make an announcement about the incident.
Shock: The referee turns round after the perpetrator makes off
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Explore more:People: Robin Van Persie, Andriy Shevchenko Places: Barcelona