In 2011, when Barcelona won their second European Cup in three seasons, it would have seemed unthinkable that Real Madrid would be the dominant team on the continent just six years later. Yet here they are, largely due to the fact that they spent money in the right areas, and Barcelona did not.

United have the resources and -- under Jose Mourinho -- the vision to become a consistent threat in the Champions League, so their new signings should be made on that basis. Mourinho's four purchases in last summer's transfer window -- Paul Pogba, Zlatan Ibrahimovic, Henrikh Mkhitaryan and Eric Bailly -- all fit that template, but the manager will be under no illusion as to the scale of work that needs to be done.

As Mourinho remarked following the Europa League final victory over Ajax, he is not here to create a poetic legacy that lasts forever -- he is here for titles. In that sense, he is somewhat similar to Zinedine Zidane, whose Madrid team is attractive to watch but does not claim to be doing anything but engaging in the business of winning.

The continent's leading club tournament presents a set of uniquely taxing demands. As such, strength in depth is vital and it was in this area that Madrid excelled this year. On the flip side, a lack of depth damned Barcelona and saw Atletico Madrid eviscerated. It is also what propelled Bayern Munich so easily past Arsenal.

It is several years since United were major players in the Champions League -- they were finalists three times in four years between 2008-11, another sign of how quickly international fortunes can change -- and so they should be expected to be a little rusty when they re-enter the competition.

Looking at their competitors, though, the Old Trafford rebuild must be comprehensive. It is one thing for Mourinho to have got the very best out of defenders such as Marcos Rojo, but the brutal reality is that Real Madrid could afford to keep James Rodriguez, the breakout star of the 2014 World Cup, out of the matchday squad altogether for last Saturday's final vs. Juventus.

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In concrete terms, what do Manchester United require? Assuming they can keep hold of David De Gea -- by no means a certainty -- they need another central midfielder who can act as a playmaker, as well as another defensive midfielder, two central defenders, a left-back, a right-back, a left-winger, and a centre-forward. That is eight players and the premium that United are obliged to pay as the world's wealthiest club means that none of them will come for much less than £30 million.

(On which note, it was disappointing to see Bernardo Silva going to local rivals Manchester City, given that he would have been ideal for United's needs. This is a loss that Mourinho may well come to regret in years to come, since there is almost no one else with Silva's qualities on the market).

At a conservative estimate, it should take at least two summer transfer windows -- given that the best players rarely change clubs in January -- for United to return to a place where they are a main threat in the later stages of the Champions League.

That is not to be unduly negative, rather to highlight the extent to which poor planning over the course of several years has hindered efforts to be effective at the highest level. Much is rightly made of the teams that won in 1999 and 2008, but it should be remembered that they were great squads as much as they were great teams. The current United options, immediately beyond the first-choice starting 11, are threadbare in key areas.

This is not all bad news. Each of Mourinho's acquisitions can be judged a success to varying degrees, with even Mkhitaryan, who endured the least enjoyable season of the quartet. providing a vital moment with the goal that sealed the Europa League win and Champions League qualification.

The manager has the funds and the nous to conduct winning business for the club -- it is just that the wait may be a while longer than many are happy to accept. But Real Madrid, their opponents in August's European Super Cup, have shown the way.