After years of speaking about value and youth, Ferguson spent £24million on a player edging towards 30, with only a year left on his contract and a dubious injury record. To an outsider, it seemed like a gamble. To Ferguson, it was anything but.
"It was a lot of money for a player in the last year of his contract," said the Scot. "But the thing about it is -- as it proved with Manchester City the year before when they paid £24million for Samir Nasri -- you either want him or you don't.
"You have to pay the price that the club are sticking out for, whether it is one year on his contract, two or three.
"We wanted him badly, so we were prepared to go the extra mile."
The rewards have been startling.
Van Persie's 20th goal of the season saved United's FA Cup hopes at West Ham on Saturday.
Four days earlier he netted twice at Wigan in a victory that kept the Red Devils seven points ahead of City in the Premier League title race, part of a current run of nine goals in nine games which has today netted him the Premier League Player of the Month award for December.
"I expected his signing would be important for us," said Ferguson.
"I really did. What he did last season was only the culmination of a few years' experience at Arsenal (he scored 37 goals in all competitions for the Gunners in 2011-12).
"Any player coming to us for the first year, you never know for sure.
"Who would have thought Wayne Rooney would have got a hat-trick on his debut? It is fairytale stuff.
"But what we did know was that we were signing a very good player."
Ferguson is also impressed by Van Persie's professionalism, which he has previously cited as an example to the remainder of his squad.
And, as Van Persie kept his cool to secure victory at Anfield in September, it is clear the Dutchman will be a man to watch on Sunday, along with Reds forward Luis Suarez, whom Ferguson insists there are no special plans for. "We pay respect to every opponent we meet," he said. "You should always give your team as much information as you can in terms of what is important in the game. Suarez is no different to any other good player we come up against."
Ferguson has Nani and Anderson available after their respective hamstring injuries, offering United additional options in both midfield and the wide positions. Wayne Rooney will not be involved though, despite reports the former Everton man has been pleading to play after recovering from the knee injury he suffered in training on Christmas Day. "He is not far away," said Ferguson. "I don't think it is an issue of guiding him along in terms of the injury he had. It is quite straightforward.
"If he starts training today, we should have him available for Wednesday's (FA Cup) replay against West Ham."
Ferguson knows what the atmosphere will be like at Old Trafford tomorrow. "It is Liverpool," said Ferguson. "A derby game. That never changes. It is always an immensely important game. It is intense and emotional. Everything you can think of in a derby game will probably be there."