Why does a loss have to be considered a setback that a fighter may potentially not recover from? It could set up a rematch or a series of rematches that could make each fighter that much bigger.
When the top fighters fight, someone has to lose. A fighter's visibility and reputation is often enhanced more by a loss in a big fight than a win over a no-name average fighter.
Look at Sugar Ray Leonard. He lost to Roberto Duran, and that was followed by a rematch a few months later that made Leonard even bigger.
Look at the Gatti/Ward trilogy. The first fight was epic, and it set up two epic rematches. Who won was almost an afterthought
Other examples of fighters benefiting from losses in big fights more than winning fights nobody cared about:
George Foreman losing to Evander Holyfield in 1991. Foreman showed he wasn't a fat joke beating up tomato cans, and the fact that he could hold his own for 12 rounds against a champ in his prime made people take him more seriously.
Roberto Duran losing a relatively close 15-round decision to Marvin Hagler in 1983. It was impressive that the naturally much smaller Duran was able to fight so competitively against a middleweight of Hagler's caliber. It set up a big fight for Duran against Thomas Hearns (he got KTFOed, but that's another story...)
Thomas Hearns getting KOed by Marvin Hagler in 1985. Hearns' performance in an all-time great fight made him that much bigger.
Ken Norton narrowly losing a rematch to Muhammad Ali in 1973 (after beating Ali earlier that year). The decision really could have gone either way. It showed that Norton's earlier win was no fluke, and established him as a top heavyweight. Norton then got a title shot against Foreman (he got KTFOed, but that's another story ... )
Razor Ruddock losing twice to Mike Tyson in 1991. The first time, Ruddock lost in a TKO on a controversial stoppage. That set up a big rematch where Ruddock went the full length of the fight and lost a decision. Ruddock enhanced his reputation by holding his own against Tyson, and this landed him a big 1992 fight against Lennox Lewis, with the winner to get a shot at the undisputed title. (He got KTFOed, but that's another story ...)
Evander Holyfield losing to Riddikk Bowe in 1992. Holyfield made a legendary, valiant effort against a younger, bigger, stronger top-tier fighter. It showed that Holyfield was a true top shelf heavyweight, and not just a blown-up cruiser weight beating up on

and B-level heavyweights.
The list goes on ...