After the pair were spotted alongside the incoming manager at Selhurst Park for Manchester United's final Premier League game of the season against Crystal Palace on Sunday, it was expected that an announcement was imminent.
On Monday, the Red Devils announced that Van der Gaag and McClaren would be joining the club to form Ten Hag's new backroom team.
McClaren, meanwhile, appointed Ten Hag as his first assistant coach at FC Twente, and the 61-year-old will be returning to Old Trafford after previously working as Sir Alex Ferguson's assistant manager until 2001.
Van der Gaag's appointment is subject to a UK Visa and a Governing Body Endorsement from the FA. body check tags ::