Sporting director Clemens Fritz brought in all three players on loan, with Bremen holding a purchase option only in Ndiaye's case. Regeer, a defensive midfielder signed from Ajax, arrived as what Fritz candidly described as an "unplanned loan," brought in specifically to cover for the long-term injury to Jens Stage.
Arthur, a Brazilian full-back on loan from Bayer Leverkusen, and Ndiaye, a Senegal international arriving from Anderlecht, both strengthen the back line.
The moves leave an obvious gap unaddressed further forward. Werder let winger Samuel Mbangula depart for good reason after an underwhelming spell, but then saw Justin Njinmah suffer a serious injury of his own that is expected to sideline him for months.
Fritz nonetheless chose not to sign a like-for-like replacement, instead passing on a move for VfB Stuttgart's Badredine Bouanani.
"We also decided against it because of the short notice and the market situation," Fritz told a media briefing on Tuesday evening.
That leaves Werder Bremen with five full-backs on the books in Arthur, Ndiaye, Olivier Deman, Mitchell Weiser and Felix Agu, but only three fit wide attacking options in Marco Grüll, Eren Dinkçi and academy prospect Paul Erevbenagie.
Fritz believes the squad can cover the shortfall internally, pointing to Weiser's ability to play in midfield and Agu's capacity to operate on the attacking flank, though the latter claim looks optimistic given Agu's game built more on pace than end product, and Weiser has been out for over a year and will need time to rediscover his best form.
The picture leaves Bremen with a familiar sense of an unfinished jigsaw, echoing last summer's business. Early deals for Kenny Quetant, Chuki, Oskar Wójcik and Dariusz Stalmach had promised a smoother window, but serious injuries to Njinmah and Stage forced a late scramble that, by Fritz's own admission, has heightened rather than eased the club's relegation anxiety heading into the new season.
Bremen's summer outgoings included a reported 700,000-euro sale involving Stalmach's arrival cost and a three-million-euro fee for Wójcik, giving some sense of the modest scale at which the club has been able to operate.
Fritz's insistence that the club achieved everything it had planned in the final days of the window will offer little comfort to supporters who watched two separate injury crises expose the thinness of Bremen's squad depth just as the transfer market was closing around them.