List boxers who got KO'd badly previously to defeating a champion.

Patterson-Johannson
Pacquiao-Marquez/Bradley

 

After being KO'd by Schmeling, Joe Louis won the Heavyweight title from James Braddock the following year.

 

Roger Mayweather lost by KO and won a 140 pound title 18 months later.

 

Hearns won titles at 168 pounds in the bouts after being knocked out by Barkley at 160.

Hearns also won titles at 154 pounds after being knocked out by Hagler at 160.

 

Zab Judah won a 140 pound title 2 fights after his KO loss to Tszyu.

 

Cory Spinks won a 154 pound title directly after his KO loss to Judah.

 

Amir khan after being ko'd by Prescott, won world title the following year in his 3rd fight since ko

 

Not exactly "KO'd badly," but examples from the past half decade-ish:
Miguel Cotto - KO'd by Margarito - defeated Clottey two fights later
Felix Sturm - TKO'd by then defeated Javier Castillejo for his title back two fights later
Kiko Martinez - TKO'd by Frampton - defeated Romero for IBF title two fights later

Not exactly one or two fights later, but sometime within a year or 3-5 fights or so:
Sonny Boy Jaro - KO'd by Segura - defeated Wonjongkam
Wladimir Klitschko - KO'd by Sanders/Brewster - defeated Byrd II
Manny Pacquiao - KO'd by Torrecampo/Singsurat - defeated Sasakul
Arthur Abraham - KO'd by then defeated Stieglitz
Adrian Hernandez - KO'd by then defeated Porpramook

If you include winning vacant titles or getting a lesser "interim" title promoted, then:
Hozumi Hasegawa - TKO'd by Montiel - defeated Burgos
John Riel Casimero - KO'd by Mthalane - defeated Lazarte/Guevara

And if we include stoppages from cuts, then we have:
Diego Corrales - TKO'd by Casamayor - defeated Casamayor
Vitali Klitschko - TKO'd by Lennox - defeated Sanders

If we include stoppages from cuts and vacant titles, then:
Moruti Mthalane - TKO'd by Donaire - defeated Miranda

 

Daniel Zaragoza recaptured his title pretty swiftly after losing it by TKO to Paul Banke, if I recall correctly.

 

Marvin Johnson lost two brutal wars to Matthew Saad Muhammad. After the 2nd one, he won the WBA title from Victor Galindez two fights later.

 

I don't think the OP expected so many examples. (The question asked by the OP is both an indication of lack of boxing knowledge, and an agenda. Just IMO)

 

Hahaha, potentially.

 

Lewis getting knocked out by Rahman in the first fight, only to get revenge in the immediate rematch.

Benn getting knocked out by Watson, but taking the WBO title off Dewitt 11 months later.

 

Duran getting KO'd by hearns and then beating Barkley