Van Persie was then the villain when his chipped spot-kick was saved, before scoring twice in the final three minutes to give his new side an unlikely three points.
"I was surprised [at Van Persie's spot-kick] because every time I have seen him take a penalty he rattles it in the corners," said Ferguson.
"He made up for it, though. He had six attempts on goal and was a real threat. But we didn't start playing until Paul Scholes came on the pitch, to be honest. I thought he changed the game right around.
Ferguson credits Scholes for win
"We worked all week on the strength of Southampton with their back-post crosses and they scored two goals from it.
"We have got to give them credit for that, they are good at it. In that period when they scored the second we were well out of it.
"Paul Scholes came on and brought composure, a consistency of passing and made the difference."
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