Without Koufax Juan Marichal becomes a three time Cy Young winner.
Gibson's reputation is about half that 1968 season, half his postseason performance. Gibson went eight innings in his first postseason start, gave up four runs. He was pulled for a pinch hitter, the Cards ended up losing 8-4. He never again failed to finish a World Series start. Game 5 of the 1964 series he gives up a game tying home run in the 9th, one out away from a shutout. He finishes. Game seven he goes into the ninth with a 7-3 lead. Yankees hit two home runs. Fuck you, he's finishing the game.
1967 is an all time performance. Three starts, three complete games, three wins. One shutout. Three runs allowed total. 26 strikeouts in 27 innings. And he hit a home run in game 7.
1968 he has that ridiculous season and strikes out 17 in game one. Ten strikeouts in game four. Imagine what an upset it was when the Tigers pulled off that win in game seven. Bottom of the 8th Gibson is down 3-0 and he hits for himself!
Gibson was probably the best pitcher in baseball from September of 1967 (when he returned from injury) through 1969.