Van de Beek confirmed Madrid were in talks with Ajax over his transfer following Saturday's 2-2 Eredivisie draw against Vitesse.
Ajax – Champions League semi-finalists last season – will face Greek side PAOK in Tuesday's opening leg of their third-round qualifier.
Asked about Van de Beek, Ten Hag told reporters: "He has come here and is going to play.
"He'll play next week too [in the second leg]."
Van de Beek made 57 appearances for Ajax last season, scoring 17 goals and supplying 13 assists as Erik ten Hag's men clinched an Eredivisie and KNVB Beker double and reached the semi-finals of the Champions League.