Mills was not one of New Zealand's first-choice seamers in the early stages of the tournament, but Hamish Bennett's injury gave him his opportunity. Mills played in three matches, but suffered a quadricep strain in the last of those games, against Canada, on March 13. He missed the quarter-final victory against South Africa, and although his injury had improved, he would not be fully fit in time to play the semifinal on March 29.
Despite not being in the XI, Mills was involved in a mid-pitch confrontation when he carried the drinks during South Africa's chase in Mirpur. Mills exchanged words with South Africa batsman Faf du Plessis following the run-out of AB de Villiers, and has been fined for breaching the ICC code of conduct.
McKay, 30, was a part of New Zealand's disastrous tour of the subcontinent late last year. He will join the squad in Colombo on March 27, two days ahead of the semi-final, where New Zealand will play the winner of the Sri Lanka v England quarter-final clash. McKay is the second replacement player in the New Zealand side, after Daryl Tuffey, who came in for Bennett.