The American goalkeeper has been sidelined since December, but returned for the Royals' reserve team on Wednesday night and came through the full 90 minutes.
But Hahnemann may have to settle for a spot on the bench due to the impressive form of Adam Federici in his absence.
Steve Coppell's side have not played for a fortnight and have since been replaced by Birmingham in second place.
Reading's last two games have ended in goalless draws so Coppell could look to freshen things up recalling Noel Hunt after the Irishman scored twice for the reserves in midweek.
Coppell has no new injury concerns to contend with and will have been encouraged to see defender Sam Sodje (knee) return to training, but Ivar Ingimarsson will miss the remainder of the season with a knee injury.
Bristol City boss Gary Johnson expects Dele Adebola to be fit for the trip.
The burly striker missed the Robins' midweek defeat at Doncaster with an ankle injury he picked up against Southampton.
"We believed Dele will be okay. There is still a lot of time left and plenty of time to select the team," said Johnson.
"Dele is the kind of player who likes to roll people, turn them and get behind defences and that is something we lacked (at Doncaster)."
Fellow forward Peter Styvar (illness) was also absent at the Keepmoat Stadium, where City saw their eight-game unbeaten run ended, and the Slovakia international remains a doubt.
Victory could see Johnson's side move into the Championship play-off places for the first time in five months.
Sporting Life