"A bit of pace, a bit of one-on-one dribbling – that's certainly still an element we're looking for," Book said in a media session on Wednesday, outlining the kind of player Borussia Dortmund hope to add.
Said El Mala of Cologne, PAOK's Giannis Konstantelias, Deportivo La Coruña's Yeremay Hernández, and free agent Jadon Sancho are the candidates currently being considered, according to Bild, with the former Dortmund winger reportedly still held in high regard at the club despite his contract with Manchester United having expired.
Doubts persist over a Sancho reunion, however. Bild reports that club officials are unsettled by the lack of concrete interest from any other side in the 26-year-old, who has been without a club for over a month since leaving Old Trafford, while there are also concerns about his fitness given he has not played consistently at a high level for some time.
Three conditions would need to be met for any move to be considered: Dortmund must have a genuine need in the position, must be convinced Sancho can contribute immediately, and the financial terms must work for both sides.
Financially, the club are only willing to offer a basic salary capped at €5m, ruling out the usual signing-on fee altogether – with the hope being that all three boxes get ticked late in the window, turning any deal into what Dortmund see as a "last-minute bargain."
El Mala, by contrast, would represent a considerably more expensive route, with Köln understood to be seeking a fee in the region of €50m for the 19-year-old, having signed him for just €350,000 little more than two years ago.
Karetsas himself arrived from Belgian side Genk for around €30m plus bonuses, giving an indication of the outlay Dortmund are prepared to make for the right profile as they look to build around their new 18-year-old playmaker.