Van Ginkel and Kalas played together at Vitesse Arnhem last season.
He told chelseafc.com: "Marco is one of the biggest talents of Holland and one the best central midfielders of his age in Europe, and I think he deserved his move the Chelsea.
"The last two years he was playing so well and without him and without Wilfried Bony we couldn't have reached the Europa League both seasons. So Marco was one of most important players in our squad so for me it is not a surprise he is here.
"His distribution is good but he is also unbelievably strong for his age and he can make runs as a No. 10 or as a No. 6 behind the strikers. He scored 11 goals last season and for a No. 6 that is really good, so he can play offensively as well."