The former Paris Saint-Germain and Monaco forward was a free agent after his contract with Al-Gharafa expired in January, with West Ham agreeing terms on a deal until the end of the season earlier this week.
But in a media conference on Friday, Allardyce revealed that Nene is not yet in condition to be considered for selection.
"Nene isn't ready physically yet," he said. "He's just started training with us, it'll be about 10 days maybe two weeks before he's ready.
"It's great to have a player like Nene, with his CV, at the club. We're all looking forward to seeing him play."
Nene was brought in as cover for Andy Carroll, who will miss the remainder of the campaign with a knee injury.
"Everyone is disappointed for Andy," Allardyce added. "Injuries happen in football, but he's had more than most.
"Hopefully he can come back fully fit and show us what he can do, because when he's fit, playing to his best, he's fantastic."
Allardyce also confirmed that the trio of Winston Reid, James Collins and Matt Jarvis are all back in training and available for Sunday's Premier League clash with Tottenham at White Hart Lane.
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