Andrew McGlashan is a senior assistant editor at ESPNcricinfo
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Milne, 22, underwent an MRI scan on Sunday after experiencing discomfort in his left heel. Mike Hesson said the problem had been an issue for two weeks but was manageable until after the quarter-final against West Indies. The amount of swelling will not allow him to play in the final week of the tournament, with New Zealand hoping to secure a first World Cup semi-final win at the seventh attempt and progress to Melbourne.
Henry joined up with New Zealand on Sunday when they arrived in Auckland, trained the day before the semi-final and was given ICC approval as a replacement late on Monday afternoon.
He was considered unlucky to miss out on the original squad after impressing in New Zealand's win over Pakistan in the UAE before Christmas. He has 21 wickets at 15.42 from eight ODI appearances and will provide competition for McClenaghan and Kyle Mills to replace Milne with the feeling growing on the eve of the semi-final that he was already the favourite.
Brendon McCullum said the team "did not have to roll out the same game plan" they have used during the tournament but added it had served them well which hinted at Henry standing a good chance of being a like-for-like replacement for Milne.
"We probably need to have another look at the wicket tomorrow, look at the overhead conditions, as well, and try and factor all those things in," McCullum said. "But I think you've seen the way we've operated with Adam in that third seamer role has worked quite well for us coming in after our two opening bowlers swinging the ball; the role that he's been able to play has been very good."
A factor in Henry's favour is that he has been playing regular cricket, albeit the red-ball version, in the Plunket Shield while the reserves in the New Zealand have been left with endless net sessions to keep themselves sharp. Since featuring in two matches against Sri Lanka in January, when acting as cover for Mills, Henry has taken 20 wickets in five matches for Canterbury including six in the most recent outing against Central Districts in Nelson. However, his numbers in the one-day Ford Trophy are far less compelling: three wickets in five matches at an economy rate of 7.29.
McClenaghan is the only player outside the first-choice XI to have appeared in the tournament, when he replaced Milne for the game against Bangladesh, but he had a poor outing as he conceded 68 runs off eight overs. Mills, meanwhile, does not provide the pace that came from Milne although the experience of 170 ODIs cannot be discounted in a World Cup semi-final.
Despite the scenario having emerged of New Zealand needing to make a change for a knockout match, McCullum stood by the decision to make as few alterations as possible during the group stage even once the team had comfortably qualified.
"I think seven from seven is a testament to the strategy that we've rolled out," he said. "We're very confident in every member of the squad that we have here, and if Matt gets brought into the squad officially, then we're obviously confident in him, as well. We talked before the tournament, as well, about the guys that sat on the periphery of the squad and how they were unlucky to obviously miss out initially.
"Matt is one of those guys who now if he does get brought in, then he has the opportunity to come out there and perform, and we'll back him immensely, just as the guys who have been sitting on the sidelines."
Milne played in six out of New Zealand's seven matches and although he only took five wickets, at a cost of 39.80, his extra pace provided a valuable contrast with the rest of the seam attack. The absence of his fielding should not be discounted, either, after he repeatedly showed his athleticism in the outfield during the tournament, especially with the catches he took against England and Afghanistan. McCullum confirmed Milne would remain with the squad during the semi-final.
"Adam has been outstanding for us in that third seamer role throughout the tournament," he said. "It's really disappointing for Adam but he'll remain part of the squad, which is good, because he's a good team man, as well, good fellow to have around."
It is not the first time Milne's body has let him down. At the start of the New Zealand season he was ruled out of the series against South Africa with an elbow injury which followed an abdominal muscle strain against India last year. Back in early 2013 he missed a one-day series in South Africa due to an Achilles problem.