18 May, 11:09
IRFU Chief Executive Philip Browne has issued a statement following confirmation of Ronan O'Gara's decision to retire from playing professional rugby.
Williams has made four changes to the starting line-up that faced Edinburgh at Murrayfield in the Magners League last Friday.
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."
Meanwhile, Harlequins boss Dean Richards has named a strong side with his players itching for action after the postponement of their Guinness Premiership game against Worcester last weekend.
England-capped scrum half Andy Gomarsall has to make do with a place on the visitors' replacements bench, while Ireland supporters will be interested to see how Corkman Mike Ross goes on the tighthead side of the 'Quins scrum.
ULSTER: 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.
HARLEQUINS: Mike Brown; Tom Williams, Gonzalo Tiesi, Jordan Turner-Hall, Ugo Monye; Nick Evans, Danny Care; Ceri Jones, Chris Brooker, Mike Ross, James Percival, George Robshaw, Will Skinner (capt), Nick Easter.
Replacements: Aston Croall, Mark Lambert, Jim Evans, Tom Guest, Andy Gomarsall, Chris Malone, Epi Taione.
Referee: Romain Poite (France)
Touch Judges: Eric Soulan, Bernard Dal Maso (both France)
Television Match Official: Jean-Christophe Gastou (France)
Match Odds (Paddy Power): Ulster to win: 6/5; Draw: 16/1; Harlequins to win: 4/6
Pre-Match Quotes: Paul Steinmetz (Ulster) - "The Edinburgh result (21-15) was a bit of a blip and it was sad after the Munster game but we're looking forward to 'Quins as they are one of the English big guns.
"They are flying high and will want to come here and pick up the points.
"There were a few stern words spoken this week after Edinburgh and it will be about how the players react and pick up from the loss. It was very frustrating for everyone and there are tough lessons to be learnt.
"We have a proud record at home in the Heineken Cup and we are very serious about this game. We want to come away with a win even though we're not in the running to make the quarter-finals.
"'Quins have a very good attacking unit but generally our defence has been getting better all season and we can take great heart from that."
Neil McMillan (Harlequins) - "Ulster's forwards are big, physical and very strong ball carriers, who are extremely effective at the breakdown.
"Their backs contain a number of very dangerous, quick runners, such as Andrew Trimble and Paddy Wallace.
"Off the pitch, they have a very vocal and passionate crowd who make it very hard for any side to go and win at their home ground.
"Although the first half of their season has been poor, as far as results are concerned, you can never underestimate them.
"They put in an impressive performance against Munster a few weeks ago and came away with a convincing win. They definitely seem to have found their form.
"Ulster are indeed a strong side, but 'Quins have proved this season that they can go away and play against the biggest names in European rugby, and in front of some of the most passionate of crowds.
"Hopefully we can put in another great Heineken Cup performance at Ravenhill this weekend."
Heineken Cup Top Scorers - Current Season: Ulster: Tries: Paddy Wallace 3; Points: Ian Humphreys 22; Harlequins: Tries: Jordan Turner-Hall, Tom Williams 3 each; Points: Chris Malone 31
Current Form - Ulster - (Magners League): Lost 16-9 at home to the Scarlets; Lost 16-15 away to the Cardiff Blues; Lost 16-14 at home to the Newport Gwent Dragons; Lost 43-0 away to the Ospreys; Won 13-9 at home to Edinburgh; (Heineken Cup): Lost 26-10 at home to Stade Francais; Lost 42-21 away to Harlequins; (Magners League): Won 22-6 at home to Munster; (Friendly): Won 62-6 at home to Portugal; (Magners League): Won 53-13 at home to Connacht; (Heineken Cup): Won 26-16 at home to the Scarlets; Drew 16-16 away to the Scarlets; (Magners League): Lost 21-13 at home to Leinster; Won 37-11 away to Munster; Lost 21-15 away to Edinburgh
Harlequins - (Guinness Premiership): Won 24-21 away to Saracens; Won 31-13 at home to Bristol; Lost 24-20 away to Gloucester; Lost 28-27 at home to London Irish; Won 30-23 away to Worcester Warriors; (EDF Energy Cup): Lost 24-23 away to the Ospreys; (Heineken Cup): Won 29-22 away to the Scarlets; Won 42-21 at home to Ulster; (EDF Energy Cup): Lost 32-17 at home to London Irish; Won 27-14 away to Worcester Warriors; (Guinness Premiership): Won 32-10 at home to London Wasps; Lost 27-14 away to Leicester Tigers; Won 21-14 at home to Bath; (Heineken Cup): Won 15-10 away to Stade Francais; Won 19-17 at home to Stade Francais; (Guinness Premiership): Lost 23-13 away to Northampton Saints; Drew 26-26 at home to Leicester Tigers; Lost 24-18 away to London Wasps
Previous European Meetings: 2
Saturday, October 18, 2008 - Pool 4 - Harlequins 42 Ulster 21, Twickenham Stoop
Wednesday, October 16, 1996 - Pool C - Ulster 15 Harlequins 21, Ravenhill
Heineken Cup Records:
2007/08: Failed to qualify from Pool 2
2006/07: Failed to qualify from Pool 5
2005/06: Failed to qualify from Pool 4
2004/05: Failed to qualify from Pool 6
2003/04: Failed to qualify from Pool 1
2002/03: Failed to qualify from Pool 6
2001/02: Failed to qualify from Pool 2
2000/01: Failed to qualify from Pool 3
1999/00: Failed to qualify from Pool 3
1997/98: Failed to qualify from Pool B
1996/97: Failed to qualify from Pool C
1995/96: Failed to qualify from Pool B
2007/08: Failed to qualify from Pool 3
2004/05: Failed to qualify from Pool 4
2001/02: Failed to qualify from Pool 4
1999/00: Failed to qualify from Pool 5
1997/98: Reached the quarter-finals
1996/97: Reached the quarter-finals
IrishRugby.ie Prediction: Ulster to win by less than 7 points