The 31-year-old missed Blackburn Rovers' 6-2 loss to Arsenal in the English Premier League last week and arrived in Dubai for the All Whites' training camp a serious doubt for the game.
The central defender, however, took part in the team's late training session on Wednesday and was given the all clear by medical staff to lead his side out at the National Stadium.
New Zealand comfortably wrapped up their six-game Oceania qualifiers last November while Bahrain had a stiffer task in Asia, beating Gulf heavyweights Saudi Arabia to qualify for the playoff.
"Ryan is a huge influence on this team," All Whites assistant coach Brian Turner said. "His presence on and off the park is immense and it would've been a body blow if he wasn't available."
Nelsen's presence will add steel to the spine of coach Ricki Herbert's team, which can also count on the experience of goalkeeper Mark Paston and midfielder Simon Elliot.
Shane Smeltz, who won the A-League's golden boot last season, and Celtic's Chris Killen form a potent partnership up front, but New Zealand may suffer from a lack of competitive international games.
"The players are now in their twenties, some of them late twenties, so there is a lot of experience," Turner added of the squad, which was built over the last four years with an eye on the 2010 campaign after Australia moved to the Asian confederation.
"In games like this you need to tap into that experience of playing in big leagues around the world because we'll need it on Saturday."
Tiny Arabian Gulf nation Bahrain, who have never qualified for the World Cup finals, will also be looking to end four years of frustration after they lost to Trinidad and Tobago in their playoff for Germany in 2006.
"You could see how upset the players and the fans were when they lost to Trinidad and Tobago four years ago," Bahrain coach Milan Macala said. "We can't lose this opportunity again.
"I don't think the team can bear it, so they'll be more determined than ever. I'd like to see even more supporters on Saturday because we need them to be louder than ever before."
Bahrain will be without suspended defender Abdulla Marzooq, who was sent off in June's win over Uzbekistan that secured the team's spot in the Asian playoff against Saudi Arabia.
The second leg is in Wellington on November 14.
