Paddy Wallace returns to inside centre with player-coach Paul Steinmetz dropping back to the replacements bench.
Cillian Willis is ruled out after fracturing his cheekbone in training earlier this week, so Ireland international Isaac Boss will start at scrum half and Belfast Harlequins clubman Paul Marshall will provide back-up on the bench.
Clinton Schifcofske also returns to the squad following his recovery from an achilles injury which sidelined him for the last two months. Mark McCrea is the player to make way.
Giving his thoughts on what is another must-win game, Williams said: "There are a number of challenges that I wanted to take on this year that were much more mental than physical, winning away from home in Europe and the Magners League is something we have just got to address.
"The field is exactly the same size, the same width and there is no reason for it. That we have never done it is not an excuse. It is completely irrelevant.
"For professionals who have beaten Munster at home, Edinburgh at home and beat a very good Llanelli side at home and then to say, 'ah, but away we can't win', is not acceptable.
"They have got to front it up. We are staying in the hotel that we like, which we go to all the time, we know our opposition, we have played them twice, won once and lost closely, and there is no reason for us to go there with any sort of baggage.
"Like I said to the guys when I was at Leinster, we couldn't win in France. Now they win in France all the time. You just have to do it.
"We are very proud of our supporters but it is not about them. It is about how the team goes out. It is 80 minutes of concentration, which is mental. It is not a physical thing. We are finishing games very strongly, our fitness is great."
ULSTER (v Scarlets): Bryn Cunningham; Andrew Trimble, Darren Cave, Paddy Wallace, Simon Danielli; Ian Humphreys, Isaac Boss; Justin Fitzpatrick, Rory Best (capt), BJ Botha, Ed O'Donoghue, Ryan Caldwell, Stephen Ferris, David Pollock, Robbie Diack.
Replacements: Nigel Brady, Tom Court, Matt McCullough, Kieron Dawson, Paul Marshall, Paul Steinmetz, Clinton Schifcofske.