Trimble has returned to the province's squad after being unavailable for the Connacht game due to a hip injury.
Fijian winger Timoci Nagusa is ruled out of Friday's game with a hamstring injury, so Williams has included young Ballymena clubman Ian Whitten in the panel.
In the forwards, Thomas Anderson returns to the squad and will be hoping to be named in the final line-up which will be confirmed on Thursday.
Injuries have ruled Carlo Del Fava (groin) and Clinton Schifcofske (knee) out of contention.
Ulster are fourth and pointless in Heineken Cup Pool 4 and desperately need a win over the Welsh side.
Speaking about Friday's big European tie, Ulster captain Rory Best said: "It did hurt us to lose against the Scarlets at Ravenhill at the start of the season and we know we have improved a lot since we played them in September.
"When we look back at that game we know we didn't start well and against good teams if you give them a lead, you will always be chasing your tail.
"This Friday is a big game for us, we know if we don't win it we are definitely out of the tournament.
"The way we are looking at it is that there is 20 points up for grabs, 20 points in the past has been enough for teams to qualify without relying on other teams so we are trying to get the full points.
"People can say what they like about it, but that is the goal we are setting ourselves. This is our quarter-final and we are going to go out and treat it as such."
Match tickets are available via www.ulsterrugby.com, the Ravenhill Ticket Line (tel: 02890 493 222, Option 2) or through the Ravenhill Ticket Office (Monday-Friday, 9am-5pm).
ULSTER Squad (v Scarlets): Backs (12) - Niall O'Connor, Bryn Cunningham, Darren Cave, Andrew Trimble, Ian Whitten, Cillian Willis, Isaac Boss, Paul Steinmetz, Ian Humphreys, Mark McCrea, Paddy Wallace, Simon Danielli.
Forwards (13) - Justin Fitzpatrick, Rory Best (capt), Nigel Brady, Tom Court, BJ Botha, Ed O'Donoghue, Ryan Caldwell, Matt McCullough, Kieron Dawson, Robbie Diack, Stephen Ferris, David Pollock, Thomas Anderson.