One fascinating part of the ongoing Edinson Cavani saga is that a certain group of Chelsea fans reacted in horror at the idea that we'd be spending huge money on the Uruguay international. And it wasn't because they were gunshy about expensive strikers, or thought that Cavani wasn't good enough*. Instead, they were deeply concerned that the acquisition of the Napoli striker would mean that Romelu Lukaku wouldn't get the chance to lead the line at Stamford Bridge next season.
*Although there are supporters in both of those camps as well, one imagines.
Lukaku's had a fascinating season. Last year, the decision to loan him out to West Bromwich Albion was an easy one. He'd stagnated at Chelsea due to a lack of playing time, and would have more opportunity to impress and grow as a player at the Hawthorns than he would have in London. Thanks to a very good year with the Baggies, the Blues are in a much tougher -- though much happier -- spot regarding what to do with the 20-year-old.
How will they make the call on whether or not Lukaku should stay or go? It's a complicated decision, so let's explore the key points.
The first and most important thing to note that loaning out Lukaku is essentially equivalent to a financial investment. When a young player is loaned out, the idea is that their development at their new club will translate into future production for the parent one -- but that means that the parent side cannot take immediate advantage of the loanee's ability.
So, the cost of the loan is an immediate loss of productivity. If Chelsea send Lukaku out to Borussia Dortmund (or whomever), they will be unable to deploy Lukaku for the duration of that loan. That hit should be obvious to everyone. The return on the investment is the chance that Lukaku would be permanently improved over his baseline (i.e. the version of Lukaku that stays at Chelsea) by getting significant playing time at a high level.
From those two statements and ignoring inflation, we can draw the following conclusion:
A loan makes sense if the value of having Romelu Lukaku on the squad in 2013/14 is less than the total value above the baseline for the remainder of his Chelsea career, including a possible sale, should he be loaned out. Or, if you want it in mathematical terms...
That looks nice and clear-cut, but there are plenty of unknowns in there. The most pressing is the unclear definition of value, which should always be thought about in terms of financial terms. What is Lukaku worth to Chelsea in 2013/14? It's not simply a matter of performance translating into cost -- if Lukaku stays and is made first choice starter, the club saves money by not acquiring a Cavani but then also incur a hit on the field as well. And if he's loaned and improves, it's entirely possible that he ends up with higher wages over his tenure at Chelsea than otherwise. Oh, and we don't know how long he'll last (x). Nor do we have a sensible idea of whether or not he'll actually improve while out on loan.
In other words, we don't have any answers, but now we have the framework with which to think rigourously about this problem. Generally speaking, for Lukaku to be worth sending out on loan, we'd have to expect him to have a long, successful career at Chelsea as well as having plenty of further development to do, or make the assumption that he's not good enough to add significant value next season (which is probably a silly assumption).
If you think Lukaku is near his peak already, the delta between loan Lukaku and baseline Lukaku is probably small enough that losing a year of him is no longer worth it. If you think he's not long for Chelsea, loaning him out makes little sense, even the club receives a reasonable transfer fee. And if you consider the loan system particularly high-risk (or you think he'll be less happy if he's sent on loan, or that he'll develop just fine at Chelsea), you'll be against the loan as well.
It's important to remember that these are mostly subjective considerations. There'll be a right answer and a wrong answer, but we don't have enough information to say for sure which is which despite having an obvious mechanism with which to make a decision.
That's why the Lukaku dilemma is so contentious. Being on one side or the other, more than most arguments in football, relies on a slew of guesses regarding player development and Lukaku's future career. There's not much firm footing to be found here, but personally I suspect that loan does make some sense. There are areas where he can definitely improve (first touch, finishing), and I'd also expect him to be around for some time yet.
But it's pretty easy to disagree with that analysis and make the case that he shouldn't be sent out again. The fact that we're in such murky water with a striker who's just turned 20 is a testament to just how far he's come since his struggles under Andre Villas-Boas. It's a good problem to have, but I certainly don't envy the people actually having to make the decision.