There were many crucial moments on Manchester United's route to securing a Premier League and Champions League double.

But the defining moment of the season came before a ball had been kicked - the day last summer when manager Sir Alex Ferguson announced he had signed not just one but four high-quality players - Owen Hargreaves, Anderson, Nani and Carlos Tevez.

United may have had a straightforward canter to the league title a year ago but Ferguson knew that to stand still would have been a serious error of judgment.

So while Chelsea, by their standards, hardly changed their squad and Arsenal trusted in their young players reaching maturity, Ferguson flashed the cash, and how.

It was also a signal that that group of young British players on whom United's success had been based for so long were now to take a back seat.

You would not say that Ryan Giggs and Paul Scholes were merely bit-part players in United's season, but they were certainly getting towards the fringes. As for Gary Neville, due to injury, barely got a kick.

Of the new arrivals, it was Tevez who had the greatest impact, scoring vital goals at vital times, including the one that beat Chelsea and last-minute equalisers against Lyon in the Champions League and Blackburn in the Premier League.

Of the other new boys, Anderson, Hargreaves and to a lesser extent Nani have allowed Ferguson a large degree of flexibility.

Critics may say that he sometimes pushed that too far, such as against Chelsea at Stamford Bridge, but overall it was a huge advantage for Ferguson, especially compared with the resources available to Arsene Wenger.

Some things though were the same as last season, not least the team's backbone of Rio Ferdinand, Nemanja Vidic, Wayne Rooney and, above everyone else, Cristiano Ronaldo.

Who would have thought that his goal tally of last season would not just be surpassed but obliterated?

It is too simplistic to say that Ronaldo is just a winger, because in Ferguson's three-pronged attack he has much more of an offensive role than being stuck out on a flank.

He is very much a forward player, and his goal contribution made an overwhelming impact on the whole season.

Interestingly, despite it being one of the best title races for years with Arsenal leading from the front and then Chelsea making a late charge, Ferguson did not usually get involved in the same kind of psychological warfare that he normally employs when the going is tight.

Whether the old veteran is mellowing in his later years - and judging by his reaction after the recent defeat at Stamford Bridge that is not the case - or that he was just so confident that his multi-talented squad were the most likely of all three challengers to last the distance.

United's double triumph sends a very clear message to Wenger and, should he stay as manager, Avram Grant that they really need to match United in both quality and strength in depth if they are to end their domestic and European superiority.

Arsenal's best XI probably supersedes United's in terms of skill and pace but they are too short of numbers and in fact their defeat at Old Trafford owed as much to players being fielded in the wrong positions - i.e Kolo Toure at right-back rather than centre-back - than to being actually outplayed on the day.

It can probably be taken for granted that Arsenal, Chelsea and Liverpool, will hit the transfer market in a big way during the summer. But what will really be intriguing is whether Ferguson does the same.

Arguably, agreeing new contracts for Wes Brown and Ferdinand are the most important pieces of transfer business he will do.

But Ferguson should also be mindful of the lesson he taught to his managerial rivals last summer: to stand still is to fall behind.