Mustard was feeding one of the bowling machines in the nets adjacent to Seddon Park, when a shot from batman Kevin Pietersen hit him in the face and he was taken to hospital with a suspected broken nose.
An England team spokesman said they would give an update on Mustard's condition later, though he would not be required to play after captain Michael Vaughan named the starting line up at a news conference.
England pace bowler Ryan Sidebottom, who had a hamstring muscle injury last week and was in doubt for the match, declared himself fit for selection after undergoing some tests and was confirmed in the team by Vaughan.
Earlier, New Zealand called in medium fast bowler Mark Gillespie as cover for Wellington team mate Iain O'Brien, who injured a toe while bowling during practice on Monday.
Gillespie, however, may not be required to occupy the third seamer role behind Kyle Mills and Chris Martin because New Zealand are considering including two spinners in their side.