I agree with Zuff, Alonso crashed, recieved bump to the head, doctors said take some time off , so he is doing. I dont think its any more sinister than that.
I doubt it, but if he is playing probabilities, (and am not deliberately being hard nosed, I really don't know how ultra dedicated sportsmen think) , but with the best will in the world, the McLaren hasnt shone in testing, so any post accident residual risk is unlikely to be met with much reward.
Australia is a power circuit, and hence is likely to suit the Mercedes chaps (and I hope the RedTeam chaps too) , so its unlikely to be a massive loss to his season especially if the Honda isnt yet on full chat. And in yr2 of these regs, and with excellant reliability from most teams in winter testing the chances of a slower car point pinching due to unreliability is lower.
So if the docs are even only advising you to take time off, not telling/demanding you do it, its probably no great loss and the least risky option.
Looking forward to the intra-team battle at Mclaren for Australia though, Mr Magnussen will be out to demonstrate that the team made the wrong call at the end of last year.