Every player 'donates' matches to their opponent when they don't play their best. Every player suffers setbacks due to injuries.
The whole point of the tennis season is to see who can perform at the highest level, over many tournaments. That's what tennis is: a test of physical and mental endurance.
If you fail the test, then you can say "if only I was fitter/stronger/less prone to injuries", but it doesn't mean anything because you've still failed. It's like reminiscing about the failed shot you hit on match point. You could have won the match, but you didn't.