The two rivals coveted the striker following his excellent season for Arsenal. However, the Red Devils won the race for his signature in a deal worth in the region of £24m.
"It's hypothetical, he was never going to City," Ferguson told reporters.
"He's been fantastic for us, a really good footballer. 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 has been in excellent form for United this season, scoring 23 goals in all competitions.