Ferguson does retain an outside hope that Rooney will be available for Wednesday's Champions League trip to CSKA Moscow, although a return for the key clash with Liverpool at Anfield on October 25 is more likely.
"Calf injuries can be troublesome," said Ferguson.
"We are trying our best to get him fit for Wednesday but I have my doubts on that. But Wayne should be fit for the Liverpool game.
"We know how difficult these things can be so we will not be taking any chances with the Moscow trip."