Organisers first said three fans had been injured but Ysern told a news conference only one was hurt, suffering a wrist injury that required further checks at a hospital.
The incident occurred during the quarter-final between Frenchman Jo-Wilfried Tsonga and Japanese fifth seed Kei Nishikori.
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"It's a plate of aluminium that was a three-metre long plate that fell on the spectators. These are plates that were above the scoreboard to protect these scoreboards from the pigeons. You know what the pigeons do, unpleasant things," said Ysern.
"Three people were sent to the public infirmary (hospital), and two of them had nothing (wrong) at all."
Ysern explained that organisers were looking for the reasons behind the incident, which happened as strong winds blew in Paris all day.
"You can't take this lightheartedly, so we are conducting an investigation as we speak," he said.
"We are going to see what's happened. We will do this with our own logistics teams, but also we will get the support from external people with whom we work."
Ysern guaranteed there would be no more such incidents.
"We have reinforced the structure, and the one that's fallen, we are not going to place it back," he said.
"But the rest has been fixed again. I mean, secured again so that there is no accident tomorrow."