Seeing as how you made the original claim, the onus is on you to provide evidence that backs it up. You gave a list of United's transfer failings, but when asked how that compares to other teams, you instead insisted your opponent disprove your claim. If you were arguing in good faith, you'd have tried to actually back up your claim.
As for you claiming your are just expressing your opinion: Saying "We've had way to many transfers fail or end up as a circus in the past few years" would be stating an opinion. "No team has had more transfers derailed or become a circus in the past few years than us" is a factual statement, and one you shouldn't be making unless you either have the data to back it up, or are willing to find the data to back it up when challenged. If you don't have the data, or can't find it, you should concede the point and move on, not try to force others to do your job for you and then declare yourself the winner when they rightly refuse to do so.