It's been my experience that bringing up an injury right away, and in a light tone makes everything much easier. If you don't say anything or try to hide it or hesitate to explain what happened, it will garner more suspicion than just walking in and saying something. I'd probably say something like "you will not believe what happened to me! I was doing xxx and pulled harder on the tool than I meant to and it flew up and hit me in the face." I'd probably kinda gesture to the wound and say "nice, huh?"

Of course, I'm a bit sarcastic...put your own spin and tone on it.