Retallick dislocated his shoulder in the Rugby Championship Test against the Springboks in Wellington and when the side for the World Cup was selected it was expected Retallick would not be available until the playoff stages of the tournament.
Foster said he couldn't understand why South Africa believed injured lock Brodie Retallick would be playing against them on Saturday.
While he was progressing favourably it was unlikely he would be available for the game, he said.
Foster did repeat earlier comments from the New Zealand camp that starting against South Africa was helpful for the side as it would give them a true measure of where they were at heading further into the World Cup.
The game itself had been on their minds for a while and especially since the win over Australia in the second Bledisloe Cup Test.
"That was a massive game for us because we were under a lot of pressure then and deservedly so. We've always said the Bledisloe's a massive trophy for us so we put a lot into that and since then we've had a bit of a break and really had a clear mindset about this tournament and the importance of game one," he said.
"It's important in game one, you want to get your performance right. We want to go and kick this tournament off at a level that we really want to put a marker down and say that's where we're at, for our own information more than anything, so that we can actually grow because you know you've got to get better and better through it.
"You know you've got to improve because everyone is going to analyse you and see what you're doing and chop and change.
"We'll deliver up our game, and we'll assess it and deliver it, we'd love to think it's off a good result, then we'll move forward," he said.
Foster said he was happy with how the side's back play had developed during the season and while it was hard to get a final gauge on it, the side had started slowly in all aspects but in the last few Tests the side had gelled and there was more understanding of how they were trying to play.
"It's not just a matter of having the same combinations, it's a matter of getting the whole squad up to speed and that's what we've been working on. That takes a bit more to get the 31 seeing the same pictures but we're pleased with the progress," he said.
Every Test required changes and adaptions and some of the nuances and adjustments were where the timing could take a little longer to get right.
"We've got to go into a game assuming, like always, that what we've seen before and what we've guessed they're going to do is the picture we're going to see but then we've got to be smart enough to adapt during it and that's the sort of stuff that we spend time on," he said.