It's a conditional. You can read the sentence as a conditional clause by moving the "if" first.
If United’s Capital One Cup capitulation at the hands of MK Dons offers a true barometer of the current state of health at Old Trafford, [then] the outlay on the Real Madrid winger may yet prove to be too much, too late.
Taking some liberties and paraphrasing out the football terms:
If you think the team's recent bad performance is indicative of their future performance, then the price was too high to pay for that new player.
In other words, the author is trying to convince you to conclude something like:
The team paid too much for the new player.
This is confirmed if we look at the article your quote comes from. We can see that the author concludes the article with:
The 'value in the market' policy has failed and it is United who are now embarking on 'kamikaze' spending.