Based on his performances for Arsenal last season, when he scored 37 goals in 48 games, Van Persie will be a great signing for United if he reproduces that form and stays fit.
The jury is out on both aspects, though, because until last season Van Persie was regularly troubled by injuries and he is approaching the age when many strikers tend to lose their edge.
Unlike defenders or midfielders, when forwards get to 31 and 32 it is much harder for them to produce the form they displayed in their twenties. Van Persie is not at that stage yet by any means, but I do think that £22 million is a lot of money for a 29-year-old striker.
Of course, he did enjoy a sensational season at Arsenal last year, and he quite rightly collected both Footballer of the Year awards. And while it is highly unusual for strike partnerships to blossom in the space of 10 seconds, you are always going to have a better chance of that happening with two class players and United now have that with Van Persie and Rooney.
Rooney has the ability to play just off the front man and slide balls around the corner for somebody such as Van Persie, who is excellent at coming short and spinning off his defender before going round the back.
So if everything goes to plan and Van Persie proves he is no longer troubled by the injuries that cursed his Arsenal career for so long until last season, United could have incredible firepower.
United’s capture of Van Persie has not forced me to change my opinion that
I still believe that this season will be a defining one for United. You can argue that they always respond well and that they have won four titles in six years, but another way of spinning the numbers tells you they have won one title in three and, by their own standards, they have been poor in those seasons.
They are still crying out for something different in the centre of midfield, although I’m sure that Sir Alex United have tried to rectify that.
Yes, they have signed the attacking midfielder Shinji Kagawa from Borussia Dortmund and he looked a fine player in the Bundesliga.
All of the noises coming out of Old Trafford suggest that he is a decent player, but impressing in Germany and translating that form to the Premier League is another matter entirely, so time will tell on Kagawa.
But United haven’t brought in a midfielder who you would say is at the top of the tree. They need a player like Paul Scholes of 15 years ago, somebody capable of scoring goals and dominating games, but those players are obviously thin on the ground.
If you took Yaya Toure of the City team and put him into the United squad, you would then say that United, and not City, would be favourites to win the title.
But as things stand, Roberto Mancini’s team still have the edge, even though they have missed a real opportunity this summer to blow everybody else away and flatten the likes of United by making two or three really big signings.
Sir Alex will look at City’s lack of big investment and see that as them leaving the door open for the likes of United to come back. You should always strengthen when on top and City are by no means the finished article, so they could have made improvements in key areas, particularly the left side of defence.
They still rely so much on Toure and Vincent Kompany and you wonder how they will cope, without further additions before the transfer deadline, if they were to lose either of those players for a length of time.
While this is a defining season for United, the same could be said of City and the Premier League because, if City win the title again, it could be a long way back for the rest.
By signing Van Persie, Ferguson’s hope will be that he has given United the goals to win the title back, but he cannot be certain of that having failed to resolve his problems in midfield.