The striker suffered an ankle injury in training, following an accidental clash with team-mate Nourdin Boukhari, and is expected to be absent until after the winter break.
The loss will be a major blow to coach Frank Rijkaard (pictured), as Koevermans has been in fine from of late, netting seven goals in 12 appearances this season.
The 23-year-old grabbed a hat-trick in the win over Willem II at the end of last month, and Rijkaard will now have to find a replacement to fill the void.
Ironically, Boukhari could be asked to step in on a temporary basis, as the Moroccan is now available for selection after serving his two-match suspension for being sent off against De Graafschap on November 24.
