The last time Rafael Nadal competed in an official match was earlier this year at the Australian Open. He was facing Mackenzie McDonald, who bested Nadal in three sets after the Spaniard injured himself while extending to get to a ball.
Since then, Nadal has been on a comeback, trying to find a way to return to the courts. He didn't do it in time for Roland Garros and eventually underwent surgery to address the issues.
The comeback will happen in about a month at the Brisbane International, after which he'll play at the Australian Open. Due to his inactivity, Nadal dropped in the ATP Rankings all the way down to 664, which is where he's ranked right now.
That's obviously not good enough to enter the Australian Open draw directly, which is why he used his protected ranking to do so. That's an option for players returning from injuries, and similarly long absences to enter a few events directly.
There was a chance for Nadal to bypass having to burn a protected ranking; however, that would have required a wild card. It's very likely that his Brisbane participation is due to a wild card entry, so the protected ranking can be used to enter a bigger event.
We'll see, but what is certain is that one of those was burned at the Australian Open. Some other players who will enter the event with a protected ranking include Marin Cilic, Denis Shapovalov, Milos Raonic, Sonwoo Kwon, Jiri Vesely, and Atillas Balazs.
It will be interesting; lots of eyeballs will be on Nadal, looking to see how he does. We shouldn't expect much, but who knows? He's Nadal, and he's surprised people before.