Koster's promotion to the top job from being head of youth at the club comes as Ajax continue to hit out at previous head coach ten Cate's impending move to Chelsea.
While this speculation continues, Ajax's technical director Martin van Geel has again hit out over ten Cate's imminent switch to Stamford Bridge to join up with coach Avram Grant.
"Henk informed us last Tuesday about this. I don't think that is right.
"For us this all comes on a very bad moment; if it was up to us we would have ended the season with Henk."
But Van Geel has generally praised ten Cate's efforts as coach at Ajax, despite the team being out of both the UEFA Champions League and Uefa Cup for this season.
He added: "We did not do well internationally, but Henk did bring in passion. Ajax play better than before, that's a fact. We scored more goals and conceded less goals than before."
Although not making any official comment, Chelsea are understood to be disappointed by van Geel's remarks especially in the light of the compensation figure which was quickly agreed between the two parties.