New Zealand's Whitelock a doubt for Rugby Championship opener
The veteran second-row forward put in a Man of the Match performance in his final game for the Canterbury Crusadersas when they beat the Waikato Chiefs to win the title last weekend and Foster said he was "pretty sore" on Wednesday.
"He didn't train today," Foster told reporters in Auckland. "Obviously, he played during that recovery period, so we just have to see how he transitions in the next week or two.
"Obviously he made a decision based on all the medical advice he got, and in many senses rolled the dice and got through it.
"But he's not right to train today so we have just got to deal with what we've got now."
New Zealand face the Pumas in Mendoza on July 8 before hosting the Springboks in their second match in the southern hemisphere test competition, which has been truncated this year because of the World Cup.
Foster selected a 36-man squad for the Rugby Championship two weeks ago with the addition of uncapped outside back Shaun Stevenson as injury cover for winger Mark Talea, who has a hamstring problem.
Another winger, Leicester Fainga'anuku, has a calf problem and Foster said Stevenson was highly likely to travel to Argentina along with uncapped prop Tevita Mafileo, who has been brought in as cover for injured tighthead Fletcher Newell.
Foster said a replacement would not be brought into the squad for centre Anton Lienert-Brown, who could miss the whole championship after being banned for a dangerous tackle during the Super Rugby final.
"There's a few bodies and injuries we're still assessing over the next couple of days," the coach added. "But no, I don't think we'll replace him."
