Karetsas, 18, has already put pen to paper on a five-year contract after arriving from Genk, filling the creative void left by Julian Brandt's free transfer to Ajax.
Dortmund are now working to land 19-year-old Said El Mala from Cologne to complete the picture, and head coach Niko Kovac appears to have identified the knock-on effect that deal would have on his system: should both new arrivals slot in behind Serhou Guirassy in Dortmund's 3-4-2-1, the two attacking midfield positions would effectively be filled, leaving last season's regular contributor Beier needing a new role.
According to the Ruhr Nachrichten, Kovac has decided Beier will now be used permanently as a wing-back, a position the 23-year-old already occupied on occasion during the closing stretch of last season with encouraging results.
Beier himself has been open about where he feels most comfortable, telling the paper: "I'm lucky that I can play in several positions.
"I also enjoy playing left-back or on the right. But for as long as I can remember I've always played further forward — that's where I feel most at home."
He is set to be given his first extended run in the role in Saturday's friendly against FC Tokyo, with Kovac explaining simply: "We're going to let him play on the wing-back line. That's the position we've planned for him tomorrow."
The reshuffle could also have consequences for Yan Couto's future. The Brazilian right-back has been open to a move away from Dortmund this summer, with Como among the clubs credited with interest, but the club had been reluctant to sanction a sale without lining up a replacement first.
Beier's repositioning appears to offer exactly that kind of internal cover, potentially speeding up any departure for the 24-year-old. Reports also emerged this week of Dortmund's interest in Eintracht Frankfurt's Elias Baum as another option at right-back, though with Beier now available on that side, pursuing a deal for the 20-year-old may no longer be considered a priority.