Fergie taunts Mancini over Van Persie deal
City boss Mancini claimed last week that van Persie's fine form for much of the season is the only reason United are on course to win the Premier League title.
Ferguson's team are 15 points clear of second-placed City heading into Monday's Manchester derby at Old Trafford and van Persie has played a key role with 19 league goals since his Â£24 million ($37 million, 28-million-euro) move from Arsenal in August.
Mancini, who has been keen to play down talk of United being vastly superior to City, thought he was going to sign van Persie himself before Ferguson stepped in to steal the Dutch striker.
And Ferguson has rubbed salt into Mancini's wounds by claiming Persie was always going to snub City once United joined the race for his signature.
Asked if City would have been stronger with van Persie, Ferguson said: "It's hypothetical, he was never going to City.
"He's been fantastic for us, a really good footballer," he was quoted as saying by British media.
"We needed a bit of maturity up front and he's been terrific.
"The improvement is not just through Robin but even when he doesn't score he's a fantastic footballer."
Van Persie's goals have dried up lately, with just one in his last 12 games, but Ferguson expects the Dutch star to rediscover his form soon.
"That will change. He will get goals," Ferguson said.
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