Williams popped a rib against Connacht last weekend and the odds are 50-50 that he will start in the match against the French side at Ravenhill.
The Ulster starting 15 will be announced on Thursday.
After making it to the final last year and losing out in the quarter finals in the previous year's competition, Mark Anscombe's side will be going in as one of the favourites.
But captain Johann Muller said any extra pressure will be disregarded by players, adding: "We just want to make sure we get the positive results."
"People say that they just expect us to get out of the pool stages, but we've got the finalists of two years ago Northampton and they're top of the league and the premiership, we've got Castres, a great French side - they beat Clemont at the weekend - and then we've got Glasgow in our pool, that we've really struggled against in the past.
"So we will have to be at our absolute best over the next couple of months if we want to get out of the pool stages and we are not taking anything for granted.
"We've got one massive hurdle on Friday night and we've got to get over that," he told UTV.
The key for us is to get a win at home straight off the bat. If we lose or drop a game early on like this it will be very hard to come back from it.
And Ulster's winning streak in the Pro12 - where they have taken five out of five of recent matches -mean expectations are high. Against Castres, Ulster centre Darren Cave said players are optimistic, but also realistic.
"There's a lot of uncertainty there, we do not know what you're going to get, and you don't know what team they're going to pick and you don't know how they're going to play and what they're going to be," he explained.
"I wouldn't say that's worrying but it's a bit different and the bottom line is it's a home game in Europe and we've got to win it."
Friday's match sees the return of South African international Ruan Pienaar, who had been playing in the Rugby Championship.
Meanwhile Ulster will play against former colleague Pedrie Wannenburg, who left the side at the end of the previous season before signing for Castres, but prop John Afoa said it will be business on the pitch.
"[Meeting Pedrie] will add a bit of spice to it and there's a couple of other foreigners from New Zealand playing in their team as well, a couple of old mates playing with them, but once we've crossed that line it's 'got to go to work', so there will be no love lost, but I'm sure we will catch up afterwards."
Ulster's 27-man squad:
Forwards: Rob Herring, Rory Best, Nigel Brady, Neil McComb, John Afoa, Tom Court, Adam Macklin, Callum Black, Johann Muller, Dan Tuohy, Lewis Stevenson, Stephen Ferris, Mike McComish, Iain Henderson, Chris Henry, Nick Williams.
Backs: Ruan Pienaar, Paul Marshall, Michael Heaney, Paddy Jackson, Paddy Wallace, Darren Cave, Luke Marshall, Andrew Trimble, Tommy Bowe, Craig Gilroy, Jared Payne.