Missing from the list of forwards is number 8 Robbie Diack, who has been ruled out of contention after sustaining a frature to the metatarsal in his right foot against Edinburgh last weekend.
Included again though is Ryan Caldwell who, given Diack's unavailability, could be making a timely return to rugby from an ankle injury.
However, despite being named in last weekend's squad to face Edinburgh, Caldwell did not make the final cut and this week, Chris Henry has also been drafted into the province's extended panel.
In the backs, Bryn Cunningham (back) returns from injury as does Simon Danielli (hamstring).
If Cunningham features against Harlequins, he will be making his 150th competitive appearance for Ulster. Fijian winger Timoci Nagusa is still unavailable for selection.
Williams will name his starting line-up and panel of replacements on Friday afternoon.
ULSTER Squad (v Harlequins): Backs (12) - Clinton Schifcofske, Bryn Cunningham, Darren Cave, Andrew Trimble, Ian Whitten, Paul Marshall, Isaac Boss, Paul Steinmetz, Ian Humphreys, Mark McCrea, Paddy Wallace, Simon Danielli.
Forwards (14) - Bryan Young, Justin Fitzpatrick, Rory Best, Nigel Brady, Tom Court, BJ Botha, Ed O'Donoghue, Matt McCullough, Kieron Dawson, Stephen Ferris, David Pollock, Chris Henry, Willie Faloon, Ryan Caldwell.