The fullback joined on a straight loan from Chelsea in January to the end of the season.
He recalled to the club's website: "Of course I felt it was an honour that a big club was looking for me. I played throughout the year in Chelsea and I was always part of the squad when the request came from Dortmund.
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"I would have liked to have flown to the training camp in Marbella, but things got complicated and as a player you can't do more than wait."
Maatsen continued: Then suddenly everything happened very quickly. The agreement between Chelsea and BVB, the flight to Germany and the first game the next day. And what can I say: it went quite well."
Chelsea are expected to demand 30m from Dortmund to make the deal permanent.