Simon Danielli replaces Mark McCrea on the left wing, while Ryan Caldwell is selected in the second row alongside Ed O'Donoghue, with Matt McCullough dropping to the replacements bench.
The experienced Kieron Dawson comes off the bench and into the side at openside, in a direct swap with David Pollock.
Meanwhile, Ballymena clubman Chris Henry has also been called up for his Heineken Cup debut and his first competititve start for the province this season. Henry replaces the injured Robbie Diack.
Not considered for Saturday's game due to injury were Diack, scrum half Cillian Willis, winger Timoci Nagusa and forward duo Thomas Anderson and Carlo Del Fava.
Commenting on the Ravenhill clash, Williams said: "We are playing against a fantastic team, but that will not worry us. They are playing for a quarter final place, they have everything to play for and we have nothing to lose.
"For our guys it's about pride, we want to make up for the defeat against Edinburgh and make qualification as hard as possible for 'Quins. They beat us earlier in the season and now we are coming back at them.
"We have a lot of injury and sickness in camp, but I feel that the starting line-up is strong and quick enough to take on 'Quins and put them through their paces.
"We need to make sure that we have great players throughout the squad so that when we are hit by injury worries, we are not hit too hard.
"Mentally the guys are really up for the match. They have given it their best all week and I am happy we are ready."
ULSTER (v Harlequins): Clinton Schifcofske; Andrew Trimble, Darren Cave, Paddy Wallace, Simon Danielli, Ian Humphreys, Isaac Boss; Tom Court, Rory Best (capt), BJ Botha, Ed O'Donoghue, Ryan Caldwell, Stephen Ferris, Kieron Dawson, Chris Henry.
Replacements: Nigel Brady, Justin Fitzpatrick, Matt McCullough, David Pollock, Paul Marshall, Paul Steinmetz, Bryn Cunningham.