McCormack pulled up sharply as he raced into a good position during the 1-1 Championship draw against Blackpool at Bloomfield Road on Saturday.
Cardiff are awaiting news on the extent of the injury but manager Dave Jones has already ruled McCormack out of his team's next match.
A serious hamstring problem could see McCormack out for four to six weeks, making him a major doubt for Scotland's Hampden double-header against Macedonia on September 5 and Holland four days later.
"Ross definitely won't play at Plymouth on Tuesday because we heard a ping," Jones said.
The 22-year-old started for Scotland during their 4-0 defeat by Norway in Oslo last Wednesday, although he was sacrificed for a defender shortly after Gary Caldwell's 33rd-minute red card.
The former Rangers and Motherwell striker scored in Scotland's previous Group Nine clash, a 2-1 victory over Iceland at Hampden in April.
The injury may also ensure McCormack stays at Cardiff when the transfer window closes at the end of this month.
City rejected an offer from Hull earlier this month for the forward, who handed in a transfer request despite signing a new four-year contract this summer.