Man Utd boss Ferguson: Ferdinand missing for two weeks
Ferdinand was forced to pull out with a thigh injury, with Ferguson claiming before the game that his £29.1million defender will be going for a scan in the morning.
However, the United boss has now put a timescale on the problem, which would make the 30-year-old a doubt for England's World Cup qualifier with Croatia on September 9.
"I would imagine Rio will be out for two weeks," said Ferguson.
Without Ferdinand, United opened their Premier League title defence by beating Birmingham 1-0 courtesy of a first-half goal from Wayne Rooney.