The Netherlands legend will replace Dick Advocaat at Alkmaar at the end of the Eredivisie season after agreeing a two-year contract.
Van Basten's immediate loyalties lie with Heerenveen, though, and he has been put in the difficult situation of coming up against the club that he will take charge of when the Europa League play-offs finish.
Heerenveen are riding on the crest of a wave heading into Wednesday's semi-final first leg, having secured fifth spot in the Eredivisie after reeling off four consecutive victories - conceding just one goal in the process - and they have only been beaten once in their last 10 matches.
AZ's poor run of form included a KNVB Cup semi-final defeat to Ajax and a Europa League quarter-final exit at the hands of Benfica, so their supporters will feel they have had more than enough cup disappointment this season.
Groningen and Vitesse will contest the other semi-final and both sides also come into Wednesday's first leg at Euroborg having experienced differing fortunes at the end of the season.
Erwin van de Looi's Groningen side recorded six wins in a row to take seventh, a run that ironically started with a 3-1 win over Vitesse.
Groningen forward Richairo Zivkovic has scored five goals in his last seven games and will be eager to continue that streak as his side look to go one better than last season, when they were also beaten at this stage by Twente.
