By milb.com's stats, Stassi was 9th in the OPS in 2016. I don't know what their qualification criterion is.
But the bigger point is how meaningful that stat really is. First, OPS is not park adjusted. Second, more than half of the AAA players are the other league. Being something like 20th best seems a lot less impressive.
Just for kicks, I looked at the last 10 players who were ninth in the IL in OPS before Stassi:
Jesus Aguilar, Matt Hague, Ernesto Mejia, Jeff Clement, Yonder Alonso, Luis Antonio Rodriguez, Michael Restovich, Chris Carter (not the one currently active), Brandon Moss, and Ryan Ludwick
Not an especially impressive bunch though there are a couple of guys we'd love for Stassi to turn into.
Currently ninth in the IL in OPS...Cameron Perkins!