The 37-year-old English midfielder joined Paris Saint-Germain on a free transfer in January on a contract that runs until the end of the season.
"You can't imagine the extent to which the world is now watching PSG and Ligue 1," ESPN quotes Thiriez as saying. "It is only the French who don't seem to realise how big this is.
"I suppose you (the media) all changed your minds after seeing Beckham's first game. I was there. I was a bit nervous to see how he would get on.
"Upon seeing his first touch, I said: 'This is good. He is a real football player. Beckham is terrific!'"
Beckham has started just one game for PSG so far.