The England striker watched from the stands at Old Trafford as United beat Tottenham Hotspur 3-1 to go two points clear of Chelsea with two games remaining.
Ferguson said Rooney, a talismanic figure for England as they prepare for the World Cup starting in South Africa in June, had picked up the injury on Thursday, adding he could not say when he might return.
"It's difficult to say, groin injuries are groin injuries and I think it probably will maybe take two to three weeks to be honest," he told Sky Sports television.
Published 24 April 2010, 18:00 IST