That's unfair to the winner if a certain fighter makes excuses about his injury after the fight. For me, it's disrespectful as it's not giving credit to the winner, if the boxer has an injury, he should be transparent with it as bettors believe he is fighting with a healthy body and with good training, so they are confident that the boxer will win.
Whatever we think, it doesn't matter to them as they are just fighting for money and they have pride, when they make excuses and fans believe on it, they will still get the respect and might still be given a chance to have a rematch on the fighter that beat them.
Boxers in today's generation are more concern on the money than the honor, so think that what we are seeing is just a game, they played it well so their career will be better and they will have a good chance of making money.