The in-form forward, who has scored eight goals in his last seven games for the Red Devils, injured his ankle during a game of 'head tennis' at United's Carrington training ground.
Rooney will therefore be unavailable for England's friendly with Austria next week and the potentially crucial Euro 2008 qualifier against Croatia at Wembley on November 21.
"I think Rooney was the man who could take England to the European championships," Ferguson told Manchester United Radio.
"It was an absolute freak accident.
"He was playing a game of head tennis and he caught his foot on the stanchion that holds up the net. It's a sickening blow, but we hope he'll be back in four weeks.
"He's been in red-hot form recently, but that's football. He'll miss our games against Blackburn and Bolton - and it's obviously a big loss for England too."