That part is weird imho, how do they know he'll need a month to recover from virus? With such things sometimes you can suddenly feel as good as new after a short while, you would expect to wait for at least week or so and see how it's going, nobody throws away a slam because of a virus. Of course, he needs matches to prepare, but most of other players will only play an exo leading to AO, it's not like there's a masters going on right now.
yes, from their words he had to rest for 10 days because of the virus, the 5 last days plus 5 next ones, but he could still have played Kooyong or Auckland before the Australian open.
Tignor's idea is that they're very careful because sometimes he tried to play when not feeling well and it harmed him for later, it's true that he said several times that from now on, he would be very careful.
And this morning there were only talkings about skipping Doha : maybe in the end they decided by cautionary principle that they would be even more careful about the AO hence the decision.
Maybe ... but still we have no certainty that everything will be OK with his knee next month because we haven't had very transparent info about that knee so far