Paul Hart played down the injury to David James prior to Portsmouth's 1-0 defeat to Stoke at the Britannia Stadium on Sunday.
The veteran goalkeeper injured himself in the warm-up and had to be replaced by Jamie Ashdown shortly before the clash in the Potteries.
The England international underwent knee surgery at the end of last season and admitted recently that the operation prevented him training for up to two days after matches.
The news poured fresh doubt on the shot-stopper's place at next summer's World Cup but Hart insists James has picked up an entirely different injury.
"He pulled a calf muscle warming up," he said. "The World Cup is in June, a calf muscle is usually about three weeks. I think he will be fit for the World Cup.
"We are managing the situation (regarding James' knee) at the moment and up until now he has been out on the pitch.
"He has to do some gym work to help him through it but he has been there and it has not been a real problem."