I'm wondering if some 'reporter' has put 2 and 2 together and taken the fact that Button dropped out early from the Challenge Fuerte Triathalon to him somehow being injured.
There are plenty of reasons why someone might drop out of a long distance run, and injury is only one of them. Maybe he came up with the sniffles, maybe he couldn't concentrate, or maybe, just maybe, from what it sounds like and what happens to a massive percentage of long distance runners - HE GOT A CRAMP! At which point, he had to stop and get to a masseuse as quickly as possible. No biggie, lasts a few moments to half an hour or more, but certainly would have no bearing on how he will drive in Spain.
My guess is this story is baseless and Mclaren are right to deny it.
http://www.autoweek.com/article/20140429/F1/140429834