Let's start with a guessing game.
How many new players do Manchester United need this summer to challenge for the Premier League title next season?
Four? Is that too conservative? Five or six? Even more?
The United squad, as it stands, is not good enough. The league table doesn't lie and they're seventh.
On top of that, Inter's official website announced that United captain Nemanja Vidic will leave for the Italian club. It's still unclear whether Rio Ferdinand, Patrice Evra and Ryan Giggs will be part of the playing staff with their contracts up in May.
Only David Moyes knows how many players he would like to sign this summer to make sure that what has happened this season isn't repeated next year.
But let's say, for the sake of argument, that he needs six. Two central midfielders, a left-back, a centre-back, a winger and one other should cover it.
Of course, Moyes isn't the only one to blame for the squad needing so much work, but that's a discussion for another time. This is where United are, needing six players to go straight into the first team.
It's a major change, and history suggests that huge overhauls don't bring instant success.
A look at the title-winning teams of the last 10 years shows that managers who only have to make minor changes are far likelier to be successful.
Sir Alex Ferguson became a master at it, bringing in one or two players each summer to keep the squad ticking over.
The team which won Ferguson's last title in 2013 was unrecognisable from the one that won his first in 1993. However, he never had to rip up a team and start again. The change was gradual over 20 years.
Even when he sanctioned the sales of Mark Hughes, Andrei Kanchelskis and Paul Ince in the summer of 1995, he didn't make a major signing. Instead, he relied on the Class of '92.
City signed three—Gael Clichy, Samir Nasri and Sergio Aguero—in the summer of 2011 before winning the league in 2012.
Chelsea signed six—Petr Cech, Ricardo Carvalho, Paulo Ferreira, Thiago, Arjen Robben and Didier Drogba—before winning it in 2005, Jose Mourinho's first season in charge.
United signed five players in the summer of 2007, but only two—Owen Hargreaves and Carlos Tevez—were regulars in the team that won the Premier League and Champions League the following May.
Managers run into problems when they try to rebuild a squad in a single summer because signing players isn't an exact science. There is no magic formula to tell you whether a signing is going to work out or not.
As an educated guess, let's say around 50 per cent of Premier League transfers are successful. That means if United sign six players in the summer, three of the problems Moyes thought he had solved will still exist 12 months later.
Liverpool make a good case study. In eight of the past 10 summers, they have signed five or more players as they chased success. Because not all of them worked out, the same issues existed summer after summer.
Even Moyes, with his notoriously rigorous scouting process, has got his fair share wrong. Per Kroldrup, for example. Or Diniyar Bilyaletdinov.
There is a hope among some United fans that everything will be alright if Moyes and the board spend big this summer. There is little doubt that the squad needs strengthening, but history tells us that only very rarely do huge changes pay off immediately.
With question marks hanging over Moyes' ability to do the job, this summer he'll be under pressure to get a lot right in a very short space of time.
And he thought the toughest challenges were behind him.