He was included in Manly's 20-strong travelling squad and will be named in coach Geoff Toovey's team later this week.
"He looks good," Toovey told a press conference at Headingley Carnegie, venue for Friday's game. "He's got through training really well.
"There was a bit of doubt whether he'd come over last week. He had to pass a fitness test, not only for his neck but he had a foot injury last month.
"It's a big boost. David's a very confident player. It was a fairly serious injury and he's really excited to be back playing football."
Williams is one of nine survivors from the Manly team that beat Leeds 28-20 in the 2009 World Club Challenge at Elland Road.
Leeds are hoping that centre Kallum Watkins will recover from a wrist injury that forced him to miss last Saturday's 20-6 defeat at Wigan but coach Brian McDermott rates former England winger Lee Smith's prospects as "slim" after he damaged his ribs at the weekend.
"I don't want to write him off," McDermott said. "He'll certainly be given until the 11th hour but he was very sore this morning so he's no looking great.
"If he's popped a rib cartilage or torn some of those muscles around that area, it's pretty serious.
"Given that he's started off this year pretty good for us and his head is in the right place, I'll give him as long as we can."
The Rhinos have an obvious replacement in Ben Jones-Bishop, who was a member of their Grand Final-winning team in October and was also in the England squad for the Four Nations Series.
"We've got a couple of options there," McDermott added. "Ben Jones-Bishop is probably the notable one.
"If it's him, I'm sure he'll be doing everything possible to keep hold of his shirt."
The Australian champions' arrival at the weekend coincided in a rise in temperatures that removes any lingering doubt over Friday's game and will be a relief to the Rugby Football League after they ignored the claims of Elland Road's undersoil heating.
Co-captain Jamie Lyon, the former St Helens centre who was named Man of Steel in 2005, told the press conference: "The weather's been pretty nice. We've had a good couple of days so far."
Leeds are expecting a 20,000 full house and McDermott is delighted to be handed some home comforts.
"We'll enjoy playing at Headingley," he said. "We're in familiar surroundings and obviously the majority of the fans will be from Leeds.
"I don't think it will give us too much of an advantage. For a one-off game, these Aussie fellas will come and give it their best shot.
"If we were playing a three-game series at the same venue possibly it would have a bearing but, for one game, they'll be ready for it."
Toovey, who will be in charge of his first competitive match after succeeding Des Hasler in the close season, insists the Sea Eagles have no problem playing at Headingley.
"It doesn't matter where we play," he said. "I think the players will be very excited about playing here in front of a hostile crowd.
"We'll really take that on board. It's a great experience for our players. Not many of our players have been over here.
"There are a handful of internationals amongst them, though, who are used to playing over here and that will help us. But, as Brian says, [it's] a one-off game [and] our players will be up for it.
"They're ready to play. They're sick and tired of running around the field and playing against nobody. They want to get out there an play some football."
McDermott said he had no issues over the appointment of Australian referee Ashley Klein, who operated in Super League before moving to the NRL.
"He did all his groundwork over here," he said. "We don't let referees affect our game."