The 31-year-old shotstopper jarred his knee in the last minute of the game as he kept out John Mullin's shot from point blank range. Jarred knees have the potential to cause fairly long-term fitness problems.
Physio Jon Fearn is as yet unsure as to whether Marcus will have to sit out of Saturday's game.
"His knee is swollen so we'll have to monitor him over the next couple of days before we'll know whether he can play at Wimbledon," he explained, speaking to readingfc.co.uk.
However, any kind of spell on the sidelines for the dependable American goalkeeper and the Royals could find themselves in a lot of trouble.
Should the injury prove to be worse than first feared, it is possible that Alan Pardew may look to bring in a 'keeper to cover in Hahnemann's absence.
Meanwhile, midfielder Kevin Watson has been told that he will be sidelined for the next four weeks. The former Rotherham man limped out of Saturday's game at Derby with an ankle injury. Whilst initial reports indicated that the injury wasn't too bad he has now been told that he has damaged ankle ligaments.