Williams has only just returned following knee surgery but has injured a disc in his lower back and was forced to sit out today's training run. A call on his availability will be made tomorrow before the team flies to Wellington.
Auckland coach Wayne Pivac acknowledged it was a blow for Williams.
"It's a semifinal and no matter what level of the game you're playing, people love playing in play-off games," Pivac said.
"So with the frustrating season he's had it's fair to say he's pretty disappointed but he's also realistic and we'll put the team first and he understands that."
Should Williams be ruled out, Andrew Van der Heijden will move into the starting XV, and Nathan Hughes will come on to the bench.
Aside from Williams' possible absence, Pivac has made two changes to the Auckland team to face Wellington.
Liaki Moli replaces Andrew Van der Heijden at lock, while 19-year-old Joe Edwards will start at No 8 in place of Nathan Hughes.
Auckland beat Wellington 43-30 at Eden Park in round nine, but Pivac said the Lions would be tougher at home.
"If you look at the competition, the games lost by most teams have been away from home, funnily enough. Everyone's got a pretty good record at home including Wellington so they'll be a lot more difficult down there I'd imagine.
"They've made a couple of changes to their backline which seems to have gone really well in past weeks with Charlie Ngatai at the back. He's got a very good kicking game so we've had to look at how we approach the game with the changes they've made and certainly we think they'll be a very tough nut to crack down there."
The Auckland team to face Wellington at Westpac Stadium on Saturday night, kick-off 7.05pm, is: Charles Piutau, Ben Lam, Malakai Fekitoa, Hadleigh Parkes, Dave Thomas, Gareth Anscombe, Alby Mathewson, Joe Edwards, Daniel Braid (c), Steven Luatua, Ali Williams/Andrew Van der Heijden, Liaki Moli, Angus Ta'avao, Tom McCartney, Pauliasi Manu. Reserves: Nathan Vella, Siosiua Halanukonuka, Andrew Van der Heijden/Nathan Hughes, Sean Polwart, Scott Paterson, Simon Hickey, Lolagi Visinia.
