Clinton Schifcofske drops out of the squad entirely with a knee injury, while Stephen Ferris, who was also an injury concern after the Stade game, is included in the extended squad.
Scotland centre Rob Dewey makes a return to the squad alongside Fijian flwinger Timoci Nagusa.
Former Ulster Academy member and Ireland 'A' winger Mark McCrea has also been named in Williams' selection.
McCrea has yet to feature in a game for Ulster this season and will be hoping to force his way into the final 22 for the must-win clash with Harlequins.
In the forwards, flanker David Pollock returns from the injury that forced him out of contention for last weekend's pool opener. Williams will confirm his team and replacements on Friday.
ULSTER Squad (v Harlequins): Backs (12) - Niall O'Connor, Bryn Cunningham, Timoci Nagusa, Darren Cave, Andrew Trimble, Rob Dewey, Cillian Willis, Isaac Boss, Paul Steinmetz, Ian Humphreys, Mark McCrea, Paddy Wallace.
Forwards (13) - Justin Fitzpatrick, Nigel Brady, Tom Court, Rory Best (capt), BJ Botha, Carlo Del Fava, Ed O'Donoghue, Ryan Caldwell, Matt McCullough, Kieron Dawson, Robbie Diack, Stephen Ferris, David Pollock.