With Manchester City losing at Swansea, United hit top spot for the first time since the middle of October thanks to Rooney's second brace in as many games. It took the striker onto 26 for the season and Ferguson believes if he can get himself another 14 in United's final 10 matches, they will not be far off clinching a 20th league title.
"If Wayne gets to 40 goals I will be absolutely delighted because we will be in business then," said Ferguson.
"The City defeat was a bonus I didn't expect but I am happy to be top because only a few weeks ago we were seven points behind them.
"That is credit to the players and the resilience of the squad because as everyone knows, we have had a lot of injuries."
West Brom boss Roy Hodgson hailed Rooney as "world-class".
Indeed, as an outsider to replace Fabio Capello as England manager, Hodgson could yet be the unfortunate recipient of Rooney's absence from the first two Euro 2012 games through suspension.
"Rooney is a world-class player and will be a major loss for England in the first two matches of Euro 2012," he said.
"We have come to terms for so long now that he won't be there that you start to think it won't matter and we will get over it.
"But I think you will find that the end of those two matches a lot of people are going to be talking about the fact Wayne Rooney was suspended and had he been able to play it would have helped England's chances."