Struff is the typical nightmare draw for Nadal in the first week in Wimbledon. But this is Barcelona...
My usual prediction to a Struff - Nadal game on clay would be:
6x0 6x(-2)
And I like Struff, but the last adjective I would use to describe him is solid. The second last is clay-courter. I am very surprised he went this far (on clay). He took out rather easily a low-ranked, qualifier, clay court specialist in Delien (I would have bet on Delien), than a guy with solid clay credentials in Goffin, and a reasonable clay courter, but on the rise player in Tsitsipas. The three results are surprising to me, given Struff's style. Of course, beating Nadal would be much more than all those three surprises put together.
So, if those three wins by Struff are not, well, accidents, and if Nadal shows up like he did on his first match, then there is an outside chance of an upset. Highly unlikely. But it would interesting to see anyway.