There will be a time for comiserating the departure of the bearded wizard, but right now, there's trophies to be won and players to be purchased, so it behooves us to quickly move on and start looking at how the Mata deal impacts Chelsea's bottom line from the perspective of financial fair play accounting (which, for Chelsea, is really the only sort of accounting that matters).
Chelsea originally purchased Mata from Valencia in the summer of 2011 for around £23.5m. He was signed to a five year deal at around £67k per week.
When Mata signed the five-year deal, the £23.5m transfer fee was amortised over the life of the contract for accounting purposes. If you've been reading our various financial analyses during this transfer window, you're likely already well-aware of the concept of amortisation by now. However, for newcomers (welcome!), amortisation is the process by which an expenditure is paid off over time on the books, and it is a uniform accounting practise employed by football clubs (i.e. it's not just something Chelsea decided to do on its own to make the books look better).*
* Note that this is just an accounting practise. In reality, Chelsea likely paid Valencia the full £23.5m immediately. Similarly, Manchester United will likely be paying Chelsea the full £37m immediately or in a few installments over the next few months.
Mata's transfer fee, then, is reduced to £4.7m annually (£23.5m spread evenly over five years).
When a player is sold, however, his entire book value becomes due. Put simply, book value is the remaining portion of the transfer fee that has not yet been accounted for on the books due to amortisation. Given that Mata had 2.5 years left on his contract, the remaining book value on his transfer fee was around £11.75m.
So, the £37m transfer fee is partially offset by the remaining £11.75m of Mata's book value. This reduces the profit to £25.25m.
Chelsea also has to account for Mata's wages during the first half of the 2013-2014 season. Mata was criminally underpaid at Chelsea, earning £67k per week. A half-season's worth of wages adds up to around £1.75m. Chelsea has to account for those wages as an expenditure, and so for the purposes of boiling everything down to one number, we'll further reduce the total profit by £1.75m.
Therefore, for the purposes of FFP accounting, Chelsea's total profit on the Mata sale is £23.5m, which ironically, is exactly what Chelsea paid for Mata.
Chelsea will record the entire £23.5m profit from the sale on the 2013-2014 financial statement (which differs from when a club purchases a player and the transfer fee is amortised over the life of the player's contract).
Given that Chelsea is in perfectly fine shape with regards to complying with UEFA's financial fair play regulations, this transfer likely had little to do with FFP issues. Of course, the £23.5m profit is nice, but had Chelsea truly been concerned with financial fair play, they likely would have sold Mata in the summer when he would count towards the 2014-15 books.
To wit, Chelsea has already recorded a £10.84m profit on the Kevin de Bruyne sale. Further, Nemanja Matic only costs £3.7m on the 2013-14 FFP books. He'll cost £7.4m in each of the next four years. In addition, both the new Premier League broadcasting deal (£25m+ increase in annual revenues) and adidas deal (£10m increase) kick in this year. Chelsea certainly does not need the money, and the Mata sale likely came down to the fact that he didn't fit in with Jose Mourinho's plans.
However, for those who are angry with Jose Mourinho (or Michael Emenalo) because Chelsea just sold its two-time player of the year to Manchester United, I would suggest that such anger is severely misplaced. As another legendary football coach once said, "If they want you to cook the dinner, at least they ought to let you shop for some of the groceries."
No one is arguing that Mata represents one of the finest ingredients in all of Europe, but that matters little if Jose Mourinho is allergic to him. Mourinho has proved time and time again that he is the best chef in Europe, so if he's not going to put Mata on his menu, well then, why let him spoil on the counter when he can fetch a nice price at the market?