It's a pitty Nadal could not get ready for South American clay swing this year. Build some confidence down there, get competitive, warm the engines. Then skip sunshine double and work his way slowly through all his usual schedule in Europe. Swapp Madrid for Barcelona. Now it feels like RG came too early and there was not enough time to wake up the beast.
However, this match against Zverev is still a head scratcher for many and all I know is Rafa won't be going down easy. If Zverev shows any sign of insecurity on the court, Rafa will bite and won't let him go. It's crucial that Rafa starts strong and sends the message to the other side (read: no weak ass spins falling in half way to baseline, but deep flat bombs instead).
I chose Zverev based on current form, but I won't be surprised if it goes the other way.