He was heading to Australia to work as a radio pundit but with numerous injury concerns, Williams, who played for the Lions in 2005 and 2009, has been seconded by Warren Gatland for Tuesday's match with the Brumbies, the last warm-up game before the first Test.
Williams will line up on one wing on Tuesday with Christian Wade on the other and Billy Twelvetrees and Brad Barritt in the centres after all three Englishmen were also added to the squad.
Faletau said: "He's just such a legend I guess and it's good to have him back.
"I was just shocked to be honest, but it's great to be able to take the field with him again.
"It's been a while since he [Williams] retired from international rugby, I haven't really spoken to him since, just a few messages now and again.
"He's still playing rugby and he'll have no problems stepping up, it's just good to have him here.
"There are two good wingers out there, so we'll just give them the ball and hopefully they'll do the rest."
Twelvetrees and Barritt were added with Gatland preferring not to risk Brian O'Driscoll and Manu Tuilagi, who are slight injury concerns, while Jonathan Davies has played more minutes than anyone else so far.
Jamie Roberts, meanwhile, is rated doubtful for the first Test after limping out of Saturday's victory over the Waratahs, clutching his right leg.
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