I think generally you should be honest, but then be honest before the match as well. Don't lose and then bring up an injury you never mentioned before.
Also I would state that even if it effected my performance, I could have lost the match anyway.
And if you are getting injured every time you lose, maybe that's the time to play it down a bit..
How to clarky - work out most or only realistic outcome, claim the opposite
Federer, Djokovic, Delpo fan (also like Nalbandian, Dimitrov, Tsonga)