It's definitely true, the clubs tend to lose the players that gave their team a little extra quality in the summer and then two weeks later they already have to start playing those preliminary matches. Still though, like I said it's been so bad for the last ten years I think there's not really an excuse.And it's also due to the fact that since the Eredivisie produces good young players, bigger leagues use it as a supermarket. It's the same issue in France, you can't really be consistently good in Europe if you are continuously rebuilding and filling the gaps with kids however talented they are because in Europe experience is key.
AZ Alkmaar got knocked out in the qualifiers last year by some club from Kazachtstan. Feyenoord didn't reach the group phase because they got knocked out by a Slovakian club, Utrecht didn't make it against a Bosnian club this year and that's just results from the last 12 months... Literally every year there's shite like that and they never seem to learn.