Otago batsman Aaron Redmond was placed on standby by New Zealand at the Twenty20 World Cup in England on Wednesday as cover for Jesse Ryder.
Ryder spent the night under observation in a London hospital after complaining of feeling unwell and was in danger of missing New Zealand's Super eight match against Ireland in Nottingham on Thursday.
New Zealand team manager Dave Currie said Ryder's condition had still not been diagnosed but it appeared he was suffering from an infection.
He said New Zealand would wait for confirmation before deciding whether to ask the International Cricket Council (ICC) to allow Redmond to officially join the team.
"He has been in considerable discomfort, and medical advice is that he would certainly not be able to play in tomorrow's game," Currie said in a statement.
"In the meantime Aaron joins the squad as a precautionary measure. However, any replacement will have to be approved by the ICC."
Redmond has played seven tests for New Zealand and was already playing cricket in Britain.
Currie also confirmed Ross Taylor would miss the match against Ireland because of a hamstring twinge while a decision on captain Daniel Vettori would be made closer to the toss.
