Harris, promoted to start at stand-off in the Test at Bolton on Saturday that saw the Kiwis clinch the series, strained a groin muscle and is regarded as doubtful for Watford this Saturday. His absence could give St Helens' Sean Long a late chance to make his mark on the series.
Despite advance claims that he was fit to play at Bolton, Great Britain are still likely to be without Adrian Morley, while Paul Newlove is also far from being a certain starter.
According to the Kiwis' stand-off, Robbie Paul, there is little possibility of them going easy now that the series is won.
"We want to win the series three-nil and be remembered as the best Kiwi side of all time," he said.
Yet the New Zealand coach, Frank Endacott, has indicated that he might use some of the players on the fringes of the first team. "But there are some very good young players with us and we would not lose anything by bringing them in," he said.
His British counterpart, Andy Goodway, has been reassured that his job is not in danger. "Good coaches don't become bad coaches because of one Test series defeat," said the Rugby League's chief executive, Neil Tunnicliffe. "They have to be given time to grow into the job."