Giverin has impressed in pre-season in a new role as a holding midfielder as he continues his development at the club he initially joined as a seven-year-old while Dutch forward van Velzen was part of the 2011 FA Youth Cup-winning squad after arriving from Ajax.
"I think that the deal has been signed off and both flew out on Thursday morning," Warren Joyce confirmed to MUTV. "It will be a tremendous experience for them having continental football at club with high demands to get promotion.
"They're going into a pressure-cooker atmosphere with fantastic fans and hopefully they can play a part in it being a successful season.
"You've seen the benefits in the past for the likes of Jonny Evans, Fraizer Campbell, Danny Simpson, Ryan Shawcross, Lee Martin, Phil Bardsley, Sylvan Ebanks-Blake and Craig Cathcart. They're just a short few years ago and you can see what it can do for them being out there.
"Hopefully, they can enjoy their time out there and develop and improve as players."
Joyce also spoke to ManUtd.com about the impact Giverin has made in pre-season after he produced an excellent performance in the recent friendly against Curzon Ashton.
"He's a clever footballer," stressed Joyce. "I thought he was the best player on the pitch by a long way against Curzon.
"He really showed his level as he's sharp, bright in everything he does and has a tremendous attitude. The kid is a good trainer and, if you look at what his real strengths are, he is technically