@CO88LER5 He was set on signing Daley Blind and Dusan Tadic; those are really his signings. Apart from that it was a three-man effort: Edwin van der Sar (CEO), Marc Overmars (Director of Football) and Erik ten Hag (Head coach) all had to agree, otherwise a transfer would not happen. Out of those three, Overmars was the most determining factor.
But, you do need to remember that when he moved to Ajax, we were halfway down a -then worst- season, so he had six risk-free months to make inventory of what was required. At Man United he essentially tried to introduce what he had built at Ajax (Lissandro Martinez, Donny van de Beek, Antony, Christian Eriksen is ex-Ajax Academy, Tyrell Malacia is a 'typical' left wing back from the Dutch school), except that he never had that half year to truly build.
But that's digging deeper into the fact that coaches from the Dutch school often overlook what is required in England: the role of a Dutch Head coach has more in common with a first team coach at an English side, (example) what Rene Meulensteen was to Alex Ferguson, or what Pepijn Lijnders is to Jurgen Klopp - and in those roles Dutch coaches can excel.