The 20-year-old would reunite with head coach Mike Tullberg, who previously worked with Wätjen in Dortmund's youth set-up before taking charge in Denmark, at one point making him captain of the club's under-19 side. That relationship is understood to have been central to getting the deal over the line, with Wätjen having managed only four senior appearances at Borussia Dortmund and spent last season on loan at second-tier Bochum, where he scored twice in 26 league outings.
Reports in Germany put the fee at around €4m, with the midfielder still under contract at Dortmund until 2028. All that remains, by most accounts, is the formality of getting him signed and unveiled in Denmark, where Europa League football awaits.
With Wätjen's exit all but done, Dortmund have reportedly already turned their attention to a replacement in Konstantinos Karetsas, the Genk midfielder they have long admired. Dortmund have chased the Greek international before without success, according to German outlet Sky, and are ready to try again with a bid worth around €35m — a figure Genk are under no pressure to accept given Karetsas is contracted at the club until the summer of 2029.
The two situations underline how quickly one departure can set off a chain reaction in the transfer market, with Dortmund using the funds and squad space freed up by Wätjen's exit to fund a more established replacement in Karetsas, who has already established himself as one of the most sought-after young talents to emerge from Belgian football in recent seasons.
It also continues a busy period of squad trimming at Dortmund, who have already sold Julien Duranville to Lyon and Cole Campbell to Elversberg in the space of little more than a week, with Wätjen set to become the third academy product to depart on a permanent basis this summer.