If it is not the Zelazny story (which it probably is), this might be "25 Crunch, Split Right on Two" by George Alec Effinger from his collection of sports science fiction stories Idle Pleasures (1983), and previously published in another collection Imaginary Numbers. The plot is summarized here as:

[A] NFL player named Eldon MacDay discovers that the most severe blows of his opponents trigger lucid memories of his wife. Physical pain allows him to relive his past. And of course, it takes a profound toll on his body and mind.

There is more to it than that, though. When he is knocked unconscious during practice, MacDay travels back to night his wife ran out and was struck by a car. However, if is knocked out during a game, he realizes that he can actually changed what happened that fateful night. So in when he is running the ball, he tries to get tackled harder and harder by the opposing players—to give himself more time knocked out, and thus more time to try to save his wife.

If it is not the Zelazny story (which it probably is), this might be "25 Crunch, Split Right on Two" by George Alec Effinger from his collection of sports science fiction stories Idle Pleasures (1983), and previously published in another collection Imaginary Numbers. The plot is summarized here as:

[A] NFL player named Eldon MacDay discovers that the most severe blows of his opponents trigger lucid memories of his wife. Physical pain allows him to relive his past. And of course, it takes a profound toll on his body and mind.

There is more to it than that, though. When he is knocked unconscious during practice, MacDay travels back to night his wife ran out and was struck by a car. However, if he is knocked out during a game, he realizes that he can actually change what happened that fateful night. So in when he is running the ball, he tries to get tackled harder and harder by the opposing players—to give himself more time knocked out, and thus more time to try to save his wife.