I would not call that a "respectful", silent protest, as it involves a hazardous act meaning to deliberately disrupt/distract from the anthem. I wouldn't call him holding a sign a silent protest or trying to mime his message with his body either.
But to take your casus on i think it Depends.
National teams:
If its during our anthem when the national team plays it would be seen as very fucking disrespectful, as that player would represent the Netherlands in that upcoming game. This is in no way representative of your nation and such a person would no doubt be barred for appearing for the national team.
More realistic scenarios like: Not singing the anthem, isn't an issue over half don't, we don't do the creepy hand on the hart thingy or face the flag either. None of these are issues.
Club games:
During normal (Club games) there is no anthem or show of Dutch flags. Even if its international games. The clubs represent local regions or cities and mostly are made up out of all kinds of nationalities. This works like this in all sports, and all international games considering clubs in Europe. If a player would burn a Dutch flag before a clubgame, we'd mainly call him an idiot, and blame him for endangering others with a hazardous act. Footballers do protest all the time, main use is putting a T-shirt under your jersey so you can show the text when you score. Literally no one bats an eye, when they do though....after all, they are just footballers.
All in all we are not really that patriotic at all in symbolism , especially compared to muricans. No pledging to a flag, no abundant anthem's everywhere, no hand on the hart, no patriotic pin in the colbert, etc etc. This doesn't mean we love our nation, our way of life and freedoms any less, we're just secure enough to not have to put up a show everytime.
As an American I've never understood the whole flag worship thing we have. We really take it too far.
Flag worship is one thing, symbolizing American culture with the flag, be it Cajuns in Louisiana or pedos in Hollywood, is totally different
We don't really have a difference between club and national games. All sports are basically national if they have a tax-payer funded stadium (ie - almost all of football and baseball). A player protesting the country while making tens of millions of dollars and kneeling in a tax payer funded stadium is a slap in the face
I disagree, he's an employee of the club, not the government.
Most EU stadiums are also tax-payer funded (some clubs even are, with dubious constructions) or some clubs just refuse to pay taxes (spanish clubs are renowned for this), still club sports are not tied into nationalism and protesting a flag is his own business, as he's employed by a club / company, not the government.
Would he burn a club flag for the club he's playing he'd be outta there though.
How would people in your country react if say a soccer player burned a Netherlands flag before a game? That would be silent protest, right?